RISC-V: Add Profiles RVA/B23S64 support.

This patch adds support for the RISC-V Profiles RVA23S64 and RVB23S64.

Version log:
Fix wrong test for rvb23s.

bfd/ChangeLog:

	* elfxx-riscv.c: New Profiles.

gas/ChangeLog:

	* testsuite/gas/riscv/attribute-rva23s.d: New test.
	* testsuite/gas/riscv/attribute-rvb23s.d: New test.