add support for -print-imm-hex for AArch64

Most immediates are printed in Aarch64InstPrinter using 'formatImm' macro,
but not all of them.

Implementation contains following rules:

- floating point immediates are always printed as decimal
- signed integer immediates are printed depends on flag settings
  (for negative values 'formatImm' macro prints the value as i.e -0x01
  which may be convenient when imm is an address or offset)
- logical immediates are always printed as hex
- the 64-bit immediate for advSIMD, encoded in "a:b:c:d:e:f:g:h" is always printed as hex
- the 64-bit immedaite in exception generation instructions like:
  brk, dcps1, dcps2, dcps3, hlt, hvc, smc, svc is always printed as hex
- the rest of immediates is printed depends on availability
  of -print-imm-hex

Signed-off-by: Maciej Gabka <maciej.gabka@arm.com>
Signed-off-by: Paul Osmialowski <pawel.osmialowski@arm.com>

Differential Revision: http://reviews.llvm.org/D16929

llvm-svn: 269446