[Triple] Add isRISCV function

This matches isARM, isThumb, isAArch64 and similar helpers. Future commits
which clean-up code that currently checks for Triple::riscv32 ||
Triple::riscv64.

Differential Revision: https://reviews.llvm.org/D54215
Patch by Simon Cook.
Test case added by Alex Bradbury.

llvm-svn: 365327