RISC-V: Promote Zaamo/Zalrsc to a when using an old binutils

RISC-V: Promote Zaamo/Zalrsc to a when using an old binutils

Binutils 2.42 and before don't support Zaamo/Zalrsc. When users specify
both Zaamo and Zalrsc, promote them to 'a' in the -march string.

This does not affect testsuite results for users with old versions of binutils.
Testcases that failed due to 'call'/isa string continue to fail after this PATCH
when using an old version of binutils.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc: Add 'a' extension to
	riscv_combine_info.

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