[builtins] Add option to always build int128 routines

32-bit targets don't build these by default, but e.g. armv7 and x86 can
build them just fine, and it's useful to have the int128 routines
available for certain applications. Add a CMake option to let us include
the int128 routines for architectures which would otherwise lack them.

Reviewed By: compnerd, MaskRay, phosek

Differential Revision: https://reviews.llvm.org/D145003