Move gdb's xmalloc and friends to new file

When "common" becomes a library, linking will cause a symbol clash,
because "xmalloc" and some related symbols are defined in that
library, libiberty, and readline.

To work around this problem, this patch moves the clashing symbols to
a new file, which is then compiled separately for both gdb and
gdbserver.

gdb/ChangeLog
2019-06-11  Tom Tromey  <tom@tromey.com>

	* common/common-utils.c (xmalloc, xrealloc, xcalloc)
	(xmalloc_failed): Move to alloc.c.
	* alloc.c: New file.
	* Makefile.in (COMMON_SFILES): Add alloc.c.

gdb/gdbserver/ChangeLog
2019-06-11  Tom Tromey  <tom@tromey.com>

	* Makefile.in (SFILES): Add alloc.c.
	(OBS): Add alloc.o.
	(IPA_OBJS): Add alloc-ipa.o.
	(alloc-ipa.o): New target.
	(%.o: ../%.c): New pattern rule.