Enable verbose error messages by default for AArch64 gas.

Enable verbose error messages by default for AArch64 gas.

gas/

	* config/tc-aarch64.c (aarch64_opts): Add new option
	"mno-verbose-error".
	(verbose_error_p): Initialize to 1.
	* doc/c-aarch64.texi (AArch64 Options): Document -mverbose-error
	and -mno-verbose-error.

gas/testsuite/

	* gas/aarch64/illegal.d: Pass -mno-verbose-error.
	* gas/aarch64/verbose-error.s: Add more verbose message testcases.
	* gas/aarch64/verbose-error.l: Ditto.