Add remainder of Em16 restrictions for AArch64 gas.

Add remainder of Em16 restrictions for AArch64 gas.

This adds the missing Em16 constraints the rest of the instructions requiring them
and also adds a testcase to test all the instructions so these are checked from
now on.

The Em16 operand constrains the valid registers to the lower 16 registers when used
with a half precision qualifier.

The list has been cross checked (by hand) through the Arm ARM version Ca.

opcodes/

	PR binutils/23192
	* aarch64-tbl.h (sqdmlal, sqdmlal2, smlsl, smlsl2, sqdmlsl, sqdmlsl2,
	mul, smull, smull2, sqdmull, sqdmull2, sqdmulh, sqrdmulh, mla, umlal,
	umlal2, mls, umlsl, umlsl2, umull, umull2, sqdmlal, sqdmlsl, sqdmull,
	sqdmulh, sqrdmulh): Use Em16.

gas/

	PR binutils/23192
	* testsuite/gas/aarch64/illegal-by-element.s: New.
	* testsuite/gas/aarch64/illegal-by-element.d: New.
	* testsuite/gas/aarch64/illegal-by-element.l: New.