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.