Omit musl's stdarg.h and stddef.h from the sysroot.

Omit musl's stdarg.h and stddef.h from the sysroot.

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.