Enable libunwind on arm platforms.

Bug: 8410085
Change-Id: I8a645ec0e04b38fc186bbc0f0cc597e31b05cc55
diff --git a/libbacktrace/Android.mk b/libbacktrace/Android.mk
index 50354d0..eb55b47 100644
--- a/libbacktrace/Android.mk
+++ b/libbacktrace/Android.mk
@@ -23,7 +23,7 @@
 	liblog \
 
 # To enable using libunwind on each arch, add it to the list below.
-ifeq ($(TARGET_ARCH),$(filter $(TARGET_ARCH),))
+ifeq ($(TARGET_ARCH),$(filter $(TARGET_ARCH),arm))
 
 #----------------------------------------------------------------------------
 # The native libbacktrace library with libunwind.