RISC-V: Add configure check for Zaamo/Zalrsc assembler support

Binutils 2.42 and before don't support Zaamo/Zalrsc. Add a configure
check to prevent emitting Zaamo/Zalrsc in the arch string when the
assember does not support it.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc
	(riscv_subset_list::to_string): Skip zaamo/zalrsc when not
	supported by the assembler.
	* config.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Add zaamo/zalrsc assmeber check.

Signed-off-by: Patrick O'Neill <patrick@rivosinc.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com> # RISC-V
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com> # RISC-V