This constant is used by clang and llvm so needs to be shared.
It can't be in RISCVTargetParser.h where it is today because that
file depends on a tablegen output.
I don't want to create a new header file so I figured RISCVISAInfo.h
could be a new home for it.
lld imports RISCVISAInfo.h and lld has a class named llvm::RISCV.
So we can't expose the llvm::RISCV namespace to lld.
Thus this patch to put it inside the RISCVISAInfo class.
Note that in order to support VLEN=32 with vscale we will need to make
RVVBitsPerBlock a dynamic value based on Zve32* vs Zve64*/V being
enabled. So at that point it would probably need to be queried from