rffi upcalls don’t over-constrain return type

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

com.oracle.truffle.r.native/fficall/src/jni/Rinternals.c

.../com/oracle/truffle/r/nodes/builtin/base/foreign/Fft.java

...ruffle/r/nodes/builtin/base/foreign/ForeignFunctions.java

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

...om/oracle/truffle/r/runtime/ffi/generic/Generic_Grid.java

...m/oracle/truffle/r/runtime/ffi/generic/Generic_Tools.java

...fi/src/com/oracle/truffle/r/runtime/ffi/jni/JNI_Call.java

...i/src/com/oracle/truffle/r/runtime/ffi/jni/JNI_Stats.java

...oracle/truffle/r/runtime/ffi/jni/TraceUpCallsAdapter.java

...untime/src/com/oracle/truffle/r/runtime/TempPathName.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

...ntime/src/com/oracle/truffle/r/runtime/ffi/StatsRFFI.java

...ime/src/com/oracle/truffle/r/runtime/ffi/UpCallsRFFI.java