[RISCV] Default to ilp32d/lp64d in RISC-V Linux

When running clang as a native compiler in RISC-V Linux the flag
-mabi=ilp32d / -mabi=lp64d is always mandatory. This change makes it the
default there.

Differential Revision: https://reviews.llvm.org/D65634

llvm-svn: 371494