acpica: pass target cflags

This fixes:

| make[1]: *** [obj/aeexec.o] Error 1
| In file included from /build/linaro/build/build/tmp-eglibc/sysroots/genericarmv7a/usr/include/features.h:402:0,
|                  from /build/linaro/build/build/tmp-eglibc/sysroots/genericarmv7a/usr/include/string.h:25,
|                  from ../../../source/include/platform/aclinux.h:115,
|                  from ../../../source/include/platform/acenv.h:150,
|                  from ../../../source/include/acpi.h:56,
|                  from ../../../source/tools/acpiexec/aecommon.h:51,
|                  from ../../../source/tools/acpiexec/aehandlers.c:44:
| /build/linaro/build/build/tmp-eglibc/sysroots/genericarmv7a/usr/include/gnu/stubs.h:7:29: fatal error: gnu/stubs-soft.h: No such file or directory
|  # include <gnu/stubs-soft.h>
|                              ^
| compilation terminated.

Signed-off-by: Koen Kooi <koen.kooi@linaro.org>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>