[GR-6923] Fix typos and minor problems, add TODOs in FFI implementation.

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

...cle/truffle/r/ffi/impl/llvm/TruffleLLVM_DownCallNode.java

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

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

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

...racle/truffle/r/ffi/impl/nfi/TruffleNFI_DownCallNode.java

...le/truffle/r/ffi/impl/nfi/TruffleNFI_UpCallsRFFIImpl.java

...l/src/com/oracle/truffle/r/ffi/impl/nodes/RfEvalNode.java

.../src/com/oracle/truffle/r/ffi/processor/FFIProcessor.java

...oracle.truffle.r.native/fficall/src/truffle_llvm/Memory.c

...e.truffle.r.native/fficall/src/truffle_llvm/call_dlopen.c

...cle.truffle.r.native/fficall/src/truffle_llvm/variables.c

com.oracle.truffle.r.native/llvm_tools/llvm-helper

.../nodes/builtin/base/foreign/CallAndExternalFunctions.java

...le/r/nodes/builtin/base/foreign/FortranAndCFunctions.java

...c/com/oracle/truffle/r/runtime/data/NativeDataAccess.java

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

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

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

...m/oracle/truffle/r/test/generate/GnuROneShotRSession.java