[RISCV] Split __builtin_riscv_xperm4/8 into separate _32 and _64 builtins.

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

Also makes the builtin names match https://github.com/riscv-non-isa/riscv-c-api-doc/pull/44.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D154681