[libc] Add initial assert definition

Summary: This patch adds a temporary `__assert_fail` and `assert` definition to make it available to internal llvm libc code. `__assert_fail` writes to fd 2 directly instead of `stderr`, using SYS_write. I have not put it in its own linux directory because this is temporary and it should be using stdio's api in the future. It does not currently print out the line number (although we could do that by stringifying `__LINE__` if reviewers wish).

Reviewers: sivachandra, gchatelet, PaulkaToast

Reviewed By: sivachandra

Subscribers: mgorny, MaskRay, tschuett, libc-commits

Differential Revision: https://reviews.llvm.org/D75420