[AArch64] Add GAS recognition for "xgene-1"

* config/tc-aarch64.c (aarch64_cpus): Add entry for "xgene-1"

This adds support for the AppliedMicro X-Gene 1 processor to the
assembler.
Fix missing-prototypes error for '_initialize_spu_nat'

This patch fixes this build error below:

../../binutils-gdb/gdb/spu-linux-nat.c:616:1: error: no previous prototype for ‘_initialize_spu_nat’ [-Werror=missing-prototypes]

gdb:

2014-01-07  Yao Qi  <yao@codesourcery.com>

	* spu-linux-nat.c (_initialize_spu_nat): Declare.