Fix libraries and RAM size in PlatformIO build

[maxgerhardt]

This fixes two things:

• the ram_size variable which is fixed at 256K. Previously PlatformIO's board definitions would have 264K there (+4K scratch x/y), which would generate a linker script with the content

MEMORY
{
    FLASH(rx) : ORIGIN = 0x10000000, LENGTH = 1044480
    RAM(rwx) : ORIGIN =  0x20000000, LENGTH = 264k
    SCRATCH_X(rwx) : ORIGIN = 0x20040000, LENGTH = 4k
    SCRATCH_Y(rwx) : ORIGIN = 0x20041000, LENGTH = 4k
}

i.e., the RAM section length would be off by 8K.

• links only against lib/libpico.a, not lib/libstdc++.a by reworking the linkerflags (no -L flag to add to the search directory and -lpico -lstdc++, only link libpico.a by full path and source libstdc++.a from compiler). The libstdc++.a has weird errors which make the the system crash & burn when attempting to use even the smallest bits of the C++ standard library, such as creating a std::string or a std::map. I am not sure why this file exists here, the Arduino IDE build also does not link against it.

Related to maxgerhardt/pio-pico-core-earlephilhower-test#3 and #436.

[earlephilhower]

I'm very surprised the binaries didn't just crash immediately with a different stdc++.a interface/structures vs. the interface used in the headers used to compile the code. Good debug!

[earlephilhower]

Actually, @maxgerhardt if I get it correctly I really need to delete that libstdc++.a file. I was experimenting w/exceptions enabled/disabled at one point but never got into it deeply. Seems maybe I have some cruft in the repo left over from those experiments...