[RISCV] Set triple based on -march flag

For RISC-V the value provided to -march should determine whether to
compile for 32- or 64-bit RISC-V irrespective of the target provided to
the Clang driver. This adds a test for this flag for RISC-V and sets the
Target architecture correctly in these cases.

Differential Revision: https://reviews.llvm.org/D54214