[libc] Add Int<> type and fix (U)Int<128> compatibility issues.

Add Int<> and Int128 types to replace the usage of __int128_t in math
functions.  Clean up to make sure that (U)Int128 and __(u)int128_t are
interchangeable in the code base.

Reviewed By: sivachandra, mikhail.ramalho

Differential Revision: https://reviews.llvm.org/D152459