[AArch64] Prefer error messages from opcodes enabled in CPU_VARIANT

[AArch64] Prefer error messages from opcodes enabled in CPU_VARIANT

Printing error messages from opcodes that would otherwise not be used
due to not being enabled is pointless when there are other
alternatives.  Prefer error messages from enabled opcodes instead.

A more ideal fix would to sort error messages earlier when deciding
which KIND to return.  That is, other fatal errors of a less serious
KIND from an enabled opcode should get priority over a more serious
KIND of a disabled opcode.

gas/ChangeLog:

2020-10-20  Siddhesh Poyarekar  <siddesh.poyarekar@arm.com>

	* config/tc-aarch64.c (output_operand_error_report): Account
	for CPU_VARIANT in error messages.
	* testsuite/gas/aarch64/illegal-sve2.l: Adjust.
	* testsuite/gas/aarch64/verbose-error.l: Adjust.