RISC-V: Add new CSR feature gate handling (RV32,H)

To support feature gate like Smstateen && H, this commit adds certain
CSR feature gate handling.  It also changes how RV32-only CSRs are
handled for cleanliness.

gas/ChangeLog:

	* config/tc-riscv.c (riscv_csr_address): Add CSR feature gate
	handling for H.  Change handling on RV32.