RISC-V: Remove configure check for zabha

This patch removes the zabha configure check since it's not a breaking change
and updates the existing zaamo/zalrsc comment.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc
	(riscv_subset_list::to_string): Remove zabha configure check
	handling and clarify zaamo/zalrsc comment.
	* config.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Remove zabha configure check.

Signed-off-by: Patrick O'Neill <patrick@rivosinc.com>