Add sigaltstack

See merge request redox-os/relibc!218

Clippy fixes

See merge request redox-os/relibc!223

This reverts commit b4c738eb.

Make errno thread local

See merge request redox-os/relibc!228

Move alloc tests to expected-output tests

See merge request redox-os/relibc!226

Use try_from in getpagesize(), add test

See merge request redox-os/relibc!225

Alloc functions cleanup

See merge request redox-os/relibc!224

Implement l64a()

See merge request redox-os/relibc!227

WIP mainly because we *should* use thread locals, but #[thread_local]
causes segfaults.

This was just my attempt at being smart, I didn't realize
`compare_exchange` returned the old value (I'm dumb!), so I thought
that if the value was 1 then it must have become 2. Normally with
small errors like these you should leave a comment explaining why, but
really, compare and *exchange* is pretty obvious. My bad.