Strip libgcc to only keep fallback symbols

We use libgcc as fallback for symbols not present in libclang_rt
builtins, however we didn't know what exact symbols were being used,
some may not be intended to fallback.

Create libgcc_stripped, which only contains unwind symbols from libgcc.

Bug: 29275768
Test: bionic-unit-tests
Change-Id: I5b349fa6138e51663bf3b67109b880b4356da8e8
diff --git a/cc/testing.go b/cc/testing.go
index 2f41de1..8d76c2f 100644
--- a/cc/testing.go
+++ b/cc/testing.go
@@ -69,6 +69,13 @@
 			src: "",
 		}
 
+		toolchain_library {
+			name: "libgcc_stripped",
+			vendor_available: true,
+			recovery_available: true,
+			src: "",
+		}
+
 		cc_library {
 			name: "libc",
 			no_libgcc: true,