aarch64: Add basic target_print_operand support for CONST_STRING

aarch64: Add basic target_print_operand support for CONST_STRING

Motivated by the need to print system register names in output
assembly, this patch adds the required logic to
`aarch64_print_operand' to accept rtxs of type CONST_STRING and
process these accordingly.

Consequently, an rtx such as:

  (set (reg/i:DI 0 x0)
         (unspec:DI [(const_string ("s3_3_c13_c2_2"))])

can now be output correctly using the following output pattern when
composing `define_insn's:

  "mrs\t%x0, %1"

gcc/ChangeLog

	* config/aarch64/aarch64.cc (aarch64_print_operand): Add
	support for CONST_STRING.