[libc,c86] Compile all of ELKS C library with C86

[ghaerr]

Somewhat amazingly, we now have a full C library for the ELKS native C86 C compiler, working on ELKS!

When used with the 8086 toolchain update ghaerr/8086-toolchain#13, almost all of the entire ELKS C library is now built and available for user program development, both from a host as well as natively on ELKS using the 8086 toolchain.

The C86 library is built very similarly to the OpenWatcom C library, using a shell script (that must be edited) to specify the 8086 toolchain root, and then a set of makefiles that build the entire library as libc/libc86.a:

$ cd elks-root/libc
$ . ./c86env.sh (edit this file first and set C86= to the 8086 toolchain repo root)
$ make -f c86.mk clean
$ make -f c86.mk
(library now at ./libc86.a)

The following source files were added:

• libc/c86/syscall.s, syscall2.s, from 8086 toolchain and updated. They will be maintained here since the C86 library is built within the ELKS C library master sources.
• libc/c86/syscall.asm, syscall2.asm (NASM versions of the above, not used and likely not to be maintained). All assembling is now performed by AS86.
• libc/c86/divmod.s - ported asm/divmod.s over
• libc/include/c86/limits.h written
• libc/include/c86/stdarg.h and stddef.h, from 8086 toolchain. All include files will be maintained here within the C library.

Not supported:

• Routines requiring 64-bit ints, which are not supported by C86. These include lltostr and ulltostr.

Not yet supported:

• signal() - requires far pointer work to pass to kernel.
• stdout is currently unbuffered, requires constructors/destructors which aren't yet implemented.
• qsort produces a strange compiler bug.
• floating point.