Do not cache CallTargets in static fields

...com/oracle/truffle/r/ffi/impl/nfi/TruffleNFI_Context.java

.../src/com/oracle/truffle/r/library/graphics/RGraphics.java

...untime/src/com/oracle/truffle/r/runtime/RCompression.java

.../com/oracle/truffle/r/runtime/conn/NativeConnections.java

...me/src/com/oracle/truffle/r/runtime/context/RContext.java

...untime/src/com/oracle/truffle/r/runtime/ffi/CallRFFI.java

...e.r.runtime/src/com/oracle/truffle/r/runtime/ffi/DLL.java

...runtime/src/com/oracle/truffle/r/runtime/ffi/DLLRFFI.java

...runtime/src/com/oracle/truffle/r/runtime/ffi/ZipRFFI.java

...me/src/com/oracle/truffle/r/runtime/rng/user/UserRNG.java