[RISCV] Force enable int128 for compiling long double routines

Summary:
For RISCV32, we must force enable int128 for compiling long double routines using the flag -fforce-enable-int128.

Related clang patch: https://reviews.llvm.org/D43105

Reviewers: asb, kito-cheng, apazos, compnerd, howard.hinnant

Reviewed By: kito-cheng

Subscribers: shiva0217, efriedma, mgorny, hintonda, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D43106

llvm-svn: 326346