Allow -mcpu=cortex-a15 for clang

Clang 3.6 (including the snapshot present in AOSP) added support for
-mcpu=cortex-a15 -- there's no need to fall back to generic armv7-a
anymore.

Change-Id: I8a582261ac35e32f8f1def1d244cf83f2096520d
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>

core/clang/arm.mk

@@ -40,6 +40,5 @@ CLANG_CONFIG_arm_UNKNOWN_CFLAGS := \
 define subst-clang-incompatible-arm-flags
   $(subst -march=armv5te,-march=armv5t,\
   $(subst -march=armv5e,-march=armv5,\
-  $(subst -mcpu=cortex-a15,-march=armv7-a,\
-  $(1))))
+  $(1)))
 endef