Functions necessary for rustc

[bjorn3]

Here is a bunch of functions that need to be defined when compiling rustc for musl:

• wcsftime (Couple of improvements #168)
• getwc (Couple of improvements #168)
• putwc (Couple of improvements #168)
• wcsxfrm (Couple of improvements #168)
• wcscoll (Couple of improvements #168)
• towupper (Couple of improvements #168)
• towlower (Couple of improvements #168)

And functions that actually need to be implemented for rustc to not crash:

• wctype (and possibly iswctype)
• btowc
• wctob
• sched_yield
• posix_madvise
• getauxval
• fcntl(F_SETLK) with start/len args (blocked on Support start and length in fcntl_lock bytecodealliance/rustix#1574)

I may be missing some as I got a frankenstein combination of musl's dynamic linker (+ part of musl libc) and eyra that I used for testing.

Edit: Moved the posix_spawn family to #167 as cargo needs actual implementations.