libbacktrace: enable -fno-omit-frame-pointer for host build.

This change makes sure -fno-omit-frame-pointer flag is not
overrided by flags set in $(LLVM_HOST_BUILD_MK).

Change-Id: I53037b0eab6bd1272cec0f5c9caa058ad1a55419
diff --git a/libbacktrace/Android.build.mk b/libbacktrace/Android.build.mk
index 8e63dab..84d07f2 100644
--- a/libbacktrace/Android.build.mk
+++ b/libbacktrace/Android.build.mk
@@ -79,8 +79,13 @@
 ifeq ($(build_type),host)
   # Only build if host builds are supported.
   ifeq ($(build_host),true)
-    LOCAL_CFLAGS += -Wno-extern-c-compat -fno-omit-frame-pointer
     include $(LLVM_HOST_BUILD_MK)
+    # -fno-omit-frame-pointer should be set for host build. Because currently
+    # libunwind can't recognize .debug_frame using dwarf version 4, and it relies
+    # on stack frame pointer to do unwinding on x86.
+    # $(LLVM_HOST_BUILD_MK) overwrites -fno-omit-frame-pointer. so the below line
+    # must be after the include.
+    LOCAL_CFLAGS += -Wno-extern-c-compat -fno-omit-frame-pointer
     include $(BUILD_HOST_$(build_target))
   endif
 endif