RISC-V: Handle combine extension in canonical ordering.

RISC-V: Handle combine extension in canonical ordering.

The crypto extension have several shorthand extensions that don't consist of any extra instructions.
Take zk for example, while the extension would imply zkn, zkr, zkt.
The 3 extensions should also combine back into zk to maintain the canonical order in isa strings.
This patch addresses the above.
And if the other extension has the same situation, you can add them in riscv_combine_info[]

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc
	(riscv_combine_info): New.
	(riscv_subset_list::handle_combine_ext): Combine back into zk to
	maintain the canonical order in isa strings.
	(riscv_subset_list::parse): Ditto.
	* config/riscv/riscv-subset.h (handle_combine_ext): New.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/predef-17.c: New test.