tools/acpi: use CROSS_COMPILE to define prefix

CROSS_COMPILE can be considered as standard definition for toolchain prefix
when cross-compiling. Use it here.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

tools/power/acpi/Makefile.config

@@ -54,9 +54,10 @@ INSTALL_SCRIPT = ${INSTALL_PROGRAM}
 # to something more interesting, like "arm-linux-".  If you want
 # to compile vs uClibc, that can be done here as well.
 CROSS = #/usr/i386-linux-uclibc/usr/bin/i386-uclibc-
-CC = $(CROSS)gcc
-LD = $(CROSS)gcc
-STRIP = $(CROSS)strip
+CROSS_COMPILE ?= $(CROSS)
+CC = $(CROSS_COMPILE)gcc
+LD = $(CROSS_COMPILE)gcc
+STRIP = $(CROSS_COMPILE)strip
 HOSTCC = gcc
 
 # check if compiler option is supported