Only apply aarch64 clang WAR to the target.

Note that the ART_TARGET_CLANG_CFLAGS_$(arch) was actually broken over a
year ago. This patch fixes that and drops the no longer needed
-DNVALGRIND for clang aarch64.

Bug: http://b/23256622
Change-Id: I749db286c0fd533aeb50744b323c1faddf951d79
1 file changed