tools: power/acpi, revert to LD = gcc

Commit 7ed1c190 (tools: fix cross-compile var clobbering) removed
setting of LD to $(CROSS_COMPILE)gcc. This broke build of acpica
(acpidump) in power/acpi:
 ld: unrecognized option '-D_LINUX'

The tools pass CFLAGS to the linker (incl. -D_LINUX), so revert this
particular change and let LD be $(CC) again. Note that the old behaviour
was a bit different, it used $(CROSS_COMPILE)gcc which was eliminated by
the commit 7ed1c190. We use $(CC) for that reason.

Fixes: 7ed1c190

 (tools: fix cross-compile var clobbering)
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: 4.16+ <stable@vger.kernel.org> # 4.16+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>