Fix usleep

Closes #144

See merge request redox-os/relibc!204

Support use of new cross compiler

See merge request redox-os/relibc!203

Fix conditional compilation of sys/mman.h

See merge request redox-os/relibc!201

Aarch64 prep redux

See merge request redox-os/relibc!202

Without this the relibc tests would break. With this the relibc tests
pass and formerly breaking builds (such as uutils -> onig_sys) pass.

Typically with libc implementations, wchar_t and co are either defined
entirely by the libc or, under libc's arrangement, by headers supplied
by the compiler.

Things like dlmalloc in relibc need these definitions from relibc itself
and that's already already furnished by relibc's stddef.h.

These additional definitions here are redundant and collide with
compiler headers - for example: onig_sys (something that uutils depends
on) breaks. Instead, this patch makes the compiler headers define
things appropriately.