[Clang][RISCV] Restrict rvv builtins with zve macros

The `zve` extension specifies the maximum ELEN for both integer and floating
point mode - defined by macro `__riscv_v_elen` and `__riscv_v_elen_fp`.
This commit restricts the functions in riscv_vector.h by the zve defined
macro-s.

Change enum `RISCVExtension` to `RISCVPredefinedMacro` since now it
contains not only extensions. Also added type alignment to it.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D112986