[libc] add uint128 implementation

[libc] add uint128 implementation

Some platforms don't support proper 128 bit integers, but some
algorithms use them, such as any that use long doubles. This patch
modifies the existing UInt class to support the necessary operators.
This does not put this new class into use, that will be in followup
patches.

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D124959