Add macros for RISCV features

[michael-roe]

Add macros that test which RISCV features have been enabled at compile time.

This patch has been tested with clang version 14.0.0 (and should also work with gcc).

clang does not currently support 128 bit integer registers (CPU_FEATURES_ARCH_RISCV128) or 128 bit floating point registers (CPU_FEATURES_COMPILED_RISCV_Q); all the other features are supported by clang 14.0.0.

These features are all either in the ratified version of the RISCV Unprivileged specification, or are ratified extensions. (the V vector extension, Zb bitmap extensions, and Zk crypto extensions were ratified in November 2021). i.e. they are all officially part of the RISCV spec.

[michael-roe]

Ok, I've updated the PR to add macros for E, I, Zfh, Zfhmin, Zksed and Zksh.

clang 14.0.0 doesn't support rv32e, so you'll never see CPU_FEATURES_COMPILED_RISCV_E set unless you're using a different compiler.

[toor1245]

looks good