[AArch64] Reject invalid immediate operands to MSR UAO
    
    In the instruction to write to the ARMv8.2 PSTATE field UAO,
    MSR UAO, #<imm>, the immediate should be either 0 or 1 but GAS accepts
    any unsigned 4-bit integer.
    
    This patch implements the constraint on the immediate, generating an
    error if the immediate operand is invalid, and adds tests for the
    illegal forms.
    
    opcodes/
    2016-01-20  Matthew Wahab  <matthew.wahab@arm.com>
    
    	* aarch64-opc.c (operand_general_constraint_met_p): Check validity
    	of MSR UAO immediate operand.
    
    gas/
    2016-01-20  Matthew Wahab  <matthew.wahab@arm.com>
    
    	* testsuite/gas/aarch64/armv8_2-a-illegal.d: New.
    	* testsuite/gas/aarch64/armv8_2-a-illegal.l: New.
    	* testsuite/gas/aarch64/armv8_2-a-illegal.s: New.
    
    Change-Id: Ibdec4967c00b1ef3be9dbc43d23b2c70d1a0b28c