[nicl] Adding -rpath option to jextract tool

[Sundararajan Athijegannathan]

Webrev:  http://cr.openjdk.java.net/~sundar/jextract_rpath_option/webrev.00/

-rpath multiple use option can be used to add one or search paths for 
shared objects.
This option is expected to be used along with -l option to add library 
names.
LibraryDependencies annotation is used to record both names and paths 
for libraries.

API changes:

NativeLibrary.load, NativeLibrary.loadLibrary are similar to 
System.load, System.loadLibrary
except for JNI "on load" initialization. Removed LibraryDependency and 
ImportUnit.

Backend changes:

If "paths" array is present in LibraryDependencies annotation, backend 
uses it to resolve library
names against it. Backend will use resolved, absolute path of the 
library with NativeLibrary.load.
If "paths" is absent or if no path has resolves a library name to an 
existing file, then backend
uses JNI library search path set using the System property 
"java.library.path".

-Sundar