[GR-6163] Avoid upcall in shutdown phase.

.../com/oracle/truffle/r/ffi/impl/llvm/TruffleLLVM_Base.java

...c/com/oracle/truffle/r/ffi/impl/managed/Managed_Base.java

...mpl/src/com/oracle/truffle/r/ffi/impl/nfi/NFIContext.java

...src/com/oracle/truffle/r/ffi/impl/nfi/NativeFunction.java

...rc/com/oracle/truffle/r/ffi/impl/nfi/TruffleNFI_Base.java

...ve/fficall/src/truffle_common/Rinternals_truffle_common.h

...acle.truffle.r.native/fficall/src/truffle_nfi/rffiutils.c

...acle.truffle.r.native/fficall/src/truffle_nfi/rffiutils.h

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

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