regcache.c: fix -Wpointer-sign

regcache.c: fix -Wpointer-sign

$ make WERROR_CFLAGS="-Wpointer-sign -Werror" regcache.o 2>&1 1>/dev/null
../../src/gdb/regcache.c: In function ‘regcache_xmalloc_1’:
../../src/gdb/regcache.c:228:2: error: pointer targets in assignment differ in signedness [-Werror=pointer-sign]
../../src/gdb/regcache.c:235:2: error: pointer targets in assignment differ in signedness [-Werror=pointer-sign]
cc1: all warnings being treated as errors

regcache->register_status is "signed char".

gdb/
2013-03-07  Pedro Alves  <palves@redhat.com>

	* regcache.c (regcache_xmalloc_1): Call XCALLOC with signed char
	for 'regcache->register_status'.