[test] Update libgcc path for Android NDK r12

As noted in 8d4bc74,
the Android NDK r12 release appends a ".x" to the libgcc directory;
it is now:

```
android-ndk-r12/toolchains/arm-linux-androideabi-4.9/prebuilt/linux-x86_64/lib/gcc/arm-linux-androideabi/4.9.x
```

Update the linker path used for the Android test suite to accomodate
for this change.

Author: modocache

test/lit.cfg

@@ -767,7 +767,7 @@ elif run_os == 'linux-androideabi':
                                 config.android_ndk_gcc_version),
                             "prebuilt", "linux-x86_64", "lib", "gcc",
                             "arm-linux-androideabi",
-                            config.android_ndk_gcc_version))
+                            "{}.x".format(config.android_ndk_gcc_version)))
     config.target_build_swift = (
         '%s -target %s -sdk %s %s -Xlinker -pie %s %s %s %s'
         % (config.swiftc, config.variant_triple, config.variant_sdk,