Intrinsics/Builtins: Make names more configurable

[PhilippvK]

As far as I can tell, the names of intrinsics are generated by concatenating the arch string (of the instruction set) and the intrinsic_name (defined by the user).

This leads to the following names being generated (for arch: xcorevalu & intrinsic_name: alu_abs):

• llvm.riscv.xcorevalu.alu.abs (LLVM-IR Intrinsic)
• __builtin_xcorevalu_alu_abs (Clang Builtin)

I see two problems here:

1. __builtin_xcorevalu_alu_abs should actually be __builtin_riscv_xcorevalu_alu_abs (easy fix)
2. If I would like to have an intrinsic called llvm.riscv.cv.alu.abs instead of llvm.riscv.xcorevalu.alu.abs there is now way to achive this without renaming the arch/intruction set name. Ideally, we should have a way to (optionally) configure the names in a more specific fashion (i.e. providing a full_name: "riscv_cv_alu_abs" or prefix: "riscv_cv" option)