[RISCV] Enable __int128_t and __uint128_t through clang flag

Summary:
If the flag -fforce-enable-int128 is passed, it will enable support for __int128_t and __uint128_t types.
This flag can then be used to build compiler-rt for RISCV32.

Reviewers: asb, kito-cheng, apazos, efriedma

Reviewed By: asb, efriedma

Subscribers: shiva0217, efriedma, jfb, dschuff, sdardis, sbc100, jgravelle-google, aheejin, rbar, johnrusso, simoncook, jordy.potman.lists, sabuasal, niosHD, cfe-commits

Differential Revision: https://reviews.llvm.org/D43105

llvm-svn: 326045