[RISCV] Add macro to imply compiler availability on RISC-V Vector intrinsics version

According to version_macro.adoc under rvv-intrinsic-doc, the compiler
should have a macro to imply such availability.

Link: https://github.com/riscv-non-isa/rvv-intrinsic-doc/blob/master/version_macro.adoc

Reviewed By: kito-cheng, asb

Differential Revision: https://reviews.llvm.org/D138930