Fix ARM and AArch64 assembler tests to work in a multi-arch environment.

Fix ARM and AArch64 assembler tests to work in a multi-arch environment.

	PR 29764
gas	* testsuite/gas/arm/cpu-cortex-a76ae.d: Add arm prefix to the -m
	option passed to objdump.
	* testsuite/gas/arm/cpu-cortex-a77.d: Likewise.
	* testsuite/gas/aarch64/cpu-cortex-a76ae.d: Add aarch64 prefix to
	the -m option passed to objdump.
	* testsuite/gas/aarch64/cpu-cortex-a77.d: Likewise.

bfd	* cpu-arm.c (scan): Accept machine names prefixed with "arm:".
	* cpu-aarch64.c (scan): Accept machine names prefixed with "aarch64:".

bin	* doc/binutils.texi (objdump): Note that the -m option supports
	the <architecture>:<machine> syntax.