RISC-V: Add h extension support

RISC-V: Add h extension support

`h` was the prefix of multi-letter extension name, but it become a
extension in later RISC-V isa spec.

Fortunately we don't have any extension really defined is prefixed
with `h`, so we can just change that.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc (riscv_ext_version_table):
	Add `h`.
	(riscv_supported_std_ext): Ditto.
	(multi_letter_subset_rank): Remove `h`.
	(riscv_subset_list::parse_std_ext): Handle `h` as single letter
	extension.
	(riscv_subset_list::parse): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/arch-18.c: New.
	* gcc.target/riscv/arch-5.c: Remove test for prefixed
	with `h`.
	* gcc.target/riscv/predef-23.c: New.