[PATCH] AArch64: add GAS support for UDF instruction

[Nick Clifton]

Hi Tamar, Hi Alex, 
> Thanks for fixing it, this looks good to me but you still need a maintainer to approve.
 
I encountered lots of new testsuite failures when I tested this patch.
At least I think that I tested it - I used "patch_v2.txt".  Was this the
correct file ?  If not then please can you point me at the correct patch ?

The failures were mostly connected with the disassembler not disassembling
correctly.  eg:

  regexp "^   0:	10000001 	adr	x1, 0 <bar>$"
  line   "   0:	10000001 	.inst	0x10000001 ; undefined"
  FAIL: gas/aarch64/adr_1

(This was from a toolchain configured as aarch64-linux-gnu).  But there were
also some internal errors as well. eg:

  failed with: </gas/testsuite/gas/aarch64/advsimd-armv8_3.s: Assembler messages:
  gas/testsuite/gas/aarch64/advsimd-armv8_3.s:60: Internal error (Segmentation fault).
  Please report this bug.>, no expected output
  FAIL: gas/aarch64/advsimd-armv8_3

Cheers
  Nick