We already use the compiler's versions of these; this just moves from
having libc #include_next them to having libc just omit them entirely,
which is simpler.
This removes the special code to define musl's include guard, however
I originally added that when I was still working out how WASI's stddef.h
would interact with other code. I believe it's no longer important.
Add at least a one-line comment on every
`#ifdef __wasilibc_unmodified_upstream` briefly indicating what its
purpose is.
While here, reenable some of the code that was previously disabled when
getenv wasn't supported, as getenv is now supported.
