Respect USE_CLANG_PLATFORM_BUILD for art builds

Without this patch, ART_TARGET_CLANG is set to false unconditionally.
It should be set to whatever USE_CLANG_PLATFORM_BUILD requests, if the
latter is set.

Change-Id: I29c4397c308a1549c2097aad2c997fe732be27ac
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>
diff --git a/build/Android.common_build.mk b/build/Android.common_build.mk
index e4be21b..55a4821 100644
--- a/build/Android.common_build.mk
+++ b/build/Android.common_build.mk
@@ -122,7 +122,11 @@
 endif
 
 # Clang on the target. Target builds use GCC by default.
+ifneq ($(USE_CLANG_PLATFORM_BUILD),)
+ART_TARGET_CLANG := $(USE_CLANG_PLATFORM_BUILD)
+else
 ART_TARGET_CLANG := false
+endif
 ART_TARGET_CLANG_arm :=
 ART_TARGET_CLANG_arm64 :=
 ART_TARGET_CLANG_mips :=