[RISCV] Use clang_builtin_alias for all RISCV vector intrinsics.

Previously we used builtin_alias for overloaded intrinsics, but
macros for the non-overloaded version. This patch changes the
non-overloaded versions to also use builtin_alias, but without
the overloadable attribute.

Reviewed By: khchen, HsiangKai

Differential Revision: https://reviews.llvm.org/D112020