Remove undefined behaviour and make compatible with apple/clang

[Naxaes]

I've made two separate commits which can be cherry-picked.

Remove non-ASM statement in naked function

Non-ASM statements in naked functions are undefined behaviour, only basic ASM statements can safely be included in naked functions. Clang won't even compile (void)rsp; statements, so I replaced them with __attribite__((unused)).

https://stackoverflow.com/a/47343399

Add missing names for Apple and clang

Apple OSes doesn't have MAP_STACK or MAP_GROWSDOWN, so I replaced them with 0. They seem to be missing the threads.h header and asm keyword, so made simple defines for them that works for both gcc and clang.

Also, clang (and probably other compilers) prepend underscore to their labels.

[Naxaes]

For this to compile on Apple, you need to have Rosetta (X86_64 emulation) that's available on Apple chips, and compile with -arch x86_64.