Remove dependencies on earlier forked GNU utilities (gcc-xtensa,
binutils-gdb-xtensa) and just use GCC sources, unmodified (except for
patches in the esp-quick-toolchain directories).

GDB works with pure GNU GCC and pure GNU binutils now.  Still warnings
galore, but tested with the example sketch in the docs.

Fix some warnings present in GCC8/9 in the IPAddress code

In AddressListIterator there was a copy constructor which simply copied
the structure bit-for-bit.  That's the default operation, so remove it
to avoid the warning there.

IPAddress, add a default copy constructor since the other copy
constructors are simply parsing from one format into a native ip_addr_t.

@d-a-v, can you give these a look over and see if they're good (since
IP stuff is really your domain).

These both use shared-ptrs to handle refcnts to allocated data, so using
the default copy constructor is fine (and has been in use for a long
time).

Make GCC think _heap_start is large enough to avoid the basic (and
incorrect) bounds-checking warnings it produces.  The size chosen is
arbitrary and does not affect the actual size of the heap in any way.

Instead of a bogus size, use an indefinite size for the heap to avoid
GCC warnings

The callback function is defined to take a (void*) as parameter, but our
templates let users use anything that fits inside sizeof(void*) to be
passed in.  Add pragmas to stop GCC warnings about this, since we
already check the size of the type will fit in the allocated space.

Manually delete the divdi3.so from the libgcc.a library by running the
updated EQT's 9.1-post script.

Exceptions are broken on all builds (GCC4.8-9.1) due to the removal of
the PROGMEM non-32b read exception handler (added in the unstable
pre3.0.0).

Build the exception code with -mforce-l32 and patch
accordingly to avoid LoadStore errors.

Apply patches to select portions of the regex lib which use _stype_
(which is now in flash).

G++ seems to throw out the section attributes for templates.  This means
that the __EXCSTR(a synonym for "PSTR()") is ignored and exception.what
strings are stored in RODATA, eating up RAM.

Workaround by using the linker to place the strings keying off their name
("*__exception_what__*").

Exception.whats are now all in __exception_what__ and can be moved by
the linker to flash.  Works aroung G++ issue with segments being lost in
templates.

Move two GCC FP support routines out of iram since they are in ROM
already, saving some add'l IRAM.  Same list as gcc 4.8.

In GCC 9 (and 8 from what I read on SO), a cast of a const int to a
function pointer (via explicit or implicit reinterpret_cast) is not a
constexpr.
````
/home/earle/Arduino/hardware/esp8266com/esp8266/cores/esp8266/umm_malloc/umm_performance.cpp:45:36: error: a reinterpret_cast is not a constant expression
   45 | int constexpr (*_rom_putc1)(int) = (int (*)(int))(void*)0x40001dcc;
      |                                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
````

Remove the constexpr, potentially increasing heap usage by 4 bytes in
debug mode.

Add builds for all 5 cross-compiles to pass CI

Per @mikee47, we miss sone add'l soft-FP routined in the linker which
makes them end up in IRAM.  Move them explicitly into flash, like a
couple others we have already done this for.

Move to C++17 and C17 standards on the compiler options.

Remove "register" from core since it is deprecated.

Remove the #define abs() and #define round() which a) overwrote the
C stdlib definitions, poorly, and b) broke the GCC core code which used
"abs" as an internal function name.

Now abs() returns an integer, and not whatever was being absoluted.
fabs() etc. can be used if users need fload/double suport.

round() returns a double now, which is basically what it was returning
in the original case since adding/subtracting by a FP.

Per discussion w/@devyte, preserve the abs() and round() functionality
via the using statement.

Disassembly of the ROM shows there are 2 params to the ets_isr_t
callback.  The first is the arg passed in, the second is a pointer to an
exception frame where you can get info about when the IRQ happened.

The Arduino IDE and the build CI don't build it without a subdir, so
make one for gdbstub's example so it's visible and tested.

Remove a duplicated, different declaration for ets_irq_attach from
ets_sys.h.  It never really even matched the other declaration in the
same header.

Update the core to IRQ handlers to fix the prototype and include the
2nd, unused frame parameter.

GCC10 now uses `-fno-common` so the eboot global variables were being
placed in IRAM.  Adjust the makefile and rebuild to fix.

Somehow the prior GCC build's -mforce32 patch wasn't applying correctly,
but I was still able to get a binary.  Fixed.  Also pulled in latest
PSTR changes in progmem.h

Rebuild eboot.elf with new sources.

Add a menu to enable GCC's built-in stack smash protection.  When a
subroutine goes past its end of stack, generate a crashdump on function
exit like:

````
GCC detected stack overrun
Stack corrupted, stack smash detected.

>>>stack>>>

ctx: cont
sp: 3fffff20 end: 3fffffc0 offset: 0000
3fffff20:  40202955 00000001 0000001c 4020287e
3fffff30:  feefeffe 000000fd 00000000 00000000
...
<<<stack<<<
````

Disabled by default because there is a small per-function code overhead
(and CPU time if the function is called very frequently and is very
small).

BearSSL and LWIP are not built using stack smash detection, yet.

Report a fake exception to have the exception decoder print the actual
faulting function.  This won't tell you where in the function the issue
happened, but it will tell you the function name first and foremost.