MX support to generate testrffi and rffi code

...gen/src/com/oracle/truffle/r/ffi/codegen/CodeGenBase.java

...src/com/oracle/truffle/r/ffi/codegen/FFITestsCodeGen.java

.../oracle/truffle/r/ffi/codegen/FFIUpCallsIndexCodeGen.java

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

...e.truffle.r.native/fficall/src/common/rffi_upcallsindex.h

....truffle.r.test.native/packages/testrffi/testrffi/R/api.R

...fle.r.test.native/packages/testrffi/testrffi/R/testrffi.R

...uffle.r.test.native/packages/testrffi/testrffi/src/init.c

...e.r.test.native/packages/testrffi/testrffi/src/init_api.h

...test.native/packages/testrffi/testrffi/src/rffiwrappers.c

...test.native/packages/testrffi/testrffi/src/rffiwrappers.h

mx.fastr/mx_fastr.py