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/cc/testing.go b/cc/testing.go
index 903f76c..dc9a59d 100644
--- a/cc/testing.go
+++ b/cc/testing.go
@@ -110,6 +110,16 @@
 		}
 
 		toolchain_library {
+			name: "libunwind",
+			defaults: ["linux_bionic_supported"],
+			vendor_available: true,
+			product_available: true,
+			recovery_available: true,
+			native_bridge_supported: true,
+			src: "",
+		}
+
+		toolchain_library {
 			name: "libclang_rt.fuzzer-arm-android",
 			vendor_available: true,
 			product_available: true,