[RISCV] Split __builtin_riscv_brev8 into _32 and _64 builtin.

Allow _32 builtin on RV64 since it only brev8+sext.w.

Part of an effort to remove 'long' to mean XLen from the builtin
interface.

Matches the proposal here https://github.com/riscv-non-isa/riscv-c-api-doc/pull/44

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D154683