Remove regcache_xmalloc

Remove regcache_xmalloc

This patch removes regcache_xmalloc in favor of plain "new".

gdb/ChangeLog
2017-09-25  Tom Tromey  <tom@tromey.com>

	* regcache.h (regcache_xmalloc): Don't declare.
	(regcache_raw_set_cached_value): Update comment.
	* regcache.c (regcache_xmalloc): Remove.
	* ppc-linux-tdep.c (ppu2spu_sniffer): Use new.
	* jit.c (jit_frame_sniffer): Use new.
	* frame.c (frame_save_as_regcache): Use new.