Add the few last things to get gcc_complete working

See merge request !146

Add Gitlab CI

See merge request !140

implement mktemp

See merge request !145

Revert openlibm install script

See merge request !143

Turns out that this only worked because I didn't clean before rebuilding, so it still had access to the old files. Sorry.

Fix openlibm & fenv and add more integer types

See merge request redox-os/relibc!141

Reuse musl's setjmp implementation

See merge request redox-os/relibc!139

Install openlibm from Makefile

See merge request redox-os/relibc!138

Fix undefined behavior

See merge request redox-os/relibc!137

Transmuting from an immutable to a mutable reference is undefined
behavior in Rust, with the exception of UnsafeCell which tells the
optimizer to not draw too many conclusions. Sadly UnsafeCell::get cannot
yet be used in statics (see https://github.com/rust-lang/rust/issues/51718),
so this works around that by making the statics into functions and
making C macros to call those.