Identify generic Builtins for use with GCC and Clang/LLVM

[jeremybennett]

The CORE-V builtin specification requires that where generic builtins exist, they should be reused for CORE-V, rather than defining custom builtins.

This issue is to track the set of builtins for which this is relevant. We can still keep CORE-V specific naming as a wrapper for these in the header.

[melonedo]

These bit manipulation builtins have target-independent equivalents in GCC, and Clang should be compatible with GCC in these builtins.

CORE-V Builtin	Target-Independent Builtin	Note
__builtin_riscv_cv_bitmanip_ff1	__builtin_ctz	Undefined for x = 0
__builtin_riscv_cv_bitmanip_fl1	__builtin_clz	Undefined for x = 0
__builtin_riscv_cv_bitmanip_clb	__builtin_clrsb	Seems to return 31 for x = 0?
__builtin_riscv_cv_bitmanip_cnt	__builtin_popcount	-
__builtin_riscv_cv_bitmanip_ror	__builtin_rotateright32	Only available in Clang