RFR: JDK-8248667 Need support for building native libraries located in the test/lib directory

[Magnus Ihse Bursie]

Chris has requested help to add native test code to the test lib. From 
the bug report:

"As part of the work for JDK-8248194 I'm adding a native method to 
LingeredApp.c. The native method will be located in:

    test/lib/jdk/test/lib/apps/libLingeredApp.c

However, currently there is only support for building native libs in 
test/jdk and test/hotspot/jtreg. Support is now also needed for test/lib"

This patch adds a simple lib. Since jtreg does not accept multiple roots 
for native libraries, I have to copy this to both hotspot and jdk native 
libs in the test image. The change in BuildTestLib.gmk is strictly not 
necessary, but allows more (unfortunately not all) of the testlib to be 
compiled, including LingeredApp, which allowed me to generate a proper 
.h file to base libLingeredApp.c on.

Bug: https://bugs.openjdk.java.net/browse/JDK-8248667
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8248667-build-testlib-native/webrev.01

/Magnus