Switch platform EH unwinder to prebuilt LLVM libunwind

For the platform libc++ STL, remove the
-Wl,--exclude-libs,libunwind_llvm.a argument, which is redundant with
the same argument in deviceGlobalLdflags.

Bug: http://b/153025717
Test: device boots
Change-Id: Idd7791d52f74aab2d5f59419fb75f841fc29a2eb
diff --git a/apex/apex_test.go b/apex/apex_test.go
index fc74672..ddbbe38 100644
--- a/apex/apex_test.go
+++ b/apex/apex_test.go
@@ -1751,10 +1751,10 @@
 
 	// ensure apex variant of c++ is linked with static unwinder
 	cm := ctx.ModuleForTests("libc++", "android_arm64_armv8-a_shared_apex29").Module().(*cc.Module)
-	ensureListContains(t, cm.Properties.AndroidMkStaticLibs, "libgcc_stripped")
+	ensureListContains(t, cm.Properties.AndroidMkStaticLibs, "libunwind")
 	// note that platform variant is not.
 	cm = ctx.ModuleForTests("libc++", "android_arm64_armv8-a_shared").Module().(*cc.Module)
-	ensureListNotContains(t, cm.Properties.AndroidMkStaticLibs, "libgcc_stripped")
+	ensureListNotContains(t, cm.Properties.AndroidMkStaticLibs, "libunwind")
 }
 
 func TestApexMinSdkVersion_ErrorIfIncompatibleStubs(t *testing.T) {