[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