[AArch64] Fix two bugs in the .inst directive

The AArch64 .inst directive was implemented using EmitIntValue, which resulted
in both $x and $d (code and data) mapping symbols being emitted at the same
address. This fixes it to only emit the $x mapping symbol.

EmitIntValue also emits the value in big-endian order when targeting big-endian
systems, but instructions are always emitted in little-endian order for
AArch64.

Differential Revision: http://reviews.llvm.org/D16349

llvm-svn: 258308