Fix "incompatible pointer type" warning in gdb/aarch64-tdep.c

Fix "incompatible pointer type" warning in gdb/aarch64-tdep.c

Fixes, with x86_64-apple-darwin15-gcc (gcc 5.3.0):

 .../src/gdb/aarch64-tdep.c: In function 'aarch64_record_load_store':
 .../src/gdb/aarch64-tdep.c:3479:67: error: passing argument 3 of 'regcache_raw_read_unsigned' from incompatible pointer type [-Werror=incompatible-pointer-types]
		       bits (aarch64_insn_r->aarch64_insn, 16, 20), &reg_rm_val);
								    ^
 In file included from .../src/gdb/regcache.h:23:0,
		  from .../src/gdb/gdbarch.h:69,
		  from .../src/gdb/defs.h:620,
		  from .../src/gdb/aarch64-tdep.c:21:
 .../src/gdb/common/common-regcache.h:60:29: note: expected 'ULONGEST * {aka long unsigned int *}' but argument is of type 'uint64_t * {aka long long unsigned int *}'
  extern enum register_status regcache_raw_read_unsigned
			      ^

gdb/ChangeLog:
2016-04-20  Pedro Alves  <palves@redhat.com>

	* aarch64-tdep.c (aarch64_record_load_store): Change type of
	'reg_rm_val' local to ULONGEST.