Fix atexit/on_exit/_Exit

[ZERICO2005]

Fixes #734

• Fixed return values from on_exit/atexit
• Fixed the exit-status passed to on_exit functions via returning from main and via exit
• Re-implemented C99 _Exit
• Adds HAS_EXIT and HAS_C99__EXIT which will only include exit() and _Exit() in crt0.S if the functions are needed (similar to HAS_ABORT)

There are other issues that this PR does not fix, such as HAS_ABORT being 0 when abort is indirectly linked. Currently this PR just patches it so HAS_ABORT and HAS_EXIT are always set to 1 to prevent linker errors for the user. More here: #736

[ZERICO2005]

There are other problems that need to be resolved. I think if lto.s does not directly link _abort, then HAS_ABORT = 0. However, lto.s calls __ZNSt3__122__libcpp_verbose_abortEPKcz which does call _abort, but since HAS_ABORT = 0 there is no way for _abort to be emitted:

$ make
[cc] src/main.cpp
[lto] obj/lto.bc
[convimg] description
[linking] bin/DEMO.obj
C:\Users\zerico\Programming\ez80\toolchain\CEdev\binutils\bin\z80-none-elf-ld.exe: C:/Users/zerico/Programming/ez80/toolchain/CEdev/lib/libcxx/libcxx.a(abort_message.cpp.o): in function `___abort_message':
abort_message.cpp:(.text.___abort_message+0x1a): undefined reference to `_abort'
C:\Users\zerico\Programming\ez80\toolchain\CEdev\binutils\bin\z80-none-elf-ld.exe: C:/Users/zerico/Programming/ez80/toolchain/CEdev/lib/libcxx/libcxx.a(verbose_abort.o): in function `__ZNSt3__122__libcpp_verbose_abortEPKcz':
(.text+0xa): undefined reference to `_abort'
mingw32-make: *** [C:\Users\zerico\Programming\ez80\toolchain\CEdev\meta\makefile.mk:354: bin/DEMO.obj] Error 1