This makes relibc more compatible with other libc implementations

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.

Add sys_timeb header

- Remove incorrect header styles
- Use export.replace where header style was previously needed
- Check compilation of tests using system gcc

Based on what musl does.

wow i am stupid for writing this code

For some reason, C handles `==` before `&`.
So `a & b == c` is the same thing as `a & (b == c)`.