native build fixes for Linux; create platform.mk from GnuR info

com.oracle.truffle.r.native/builtinlibs/Makefile

com.oracle.truffle.r.native/fficall/jni/Makefile

com.oracle.truffle.r.native/gnur/Makefile

com.oracle.truffle.r.native/gnur/Makefile.gnur

com.oracle.truffle.r.native/gnur/Makefile.libs

com.oracle.truffle.r.native/gnur/Makefile.platform

com.oracle.truffle.r.native/gnur/edMakeconf

com.oracle.truffle.r.native/gnur/sedMakeconf

com.oracle.truffle.r.native/library/lib.mk

....truffle.r.test.native/packages/testrffi/src/src/Makefile

com.oracle.truffle.r.test.native/urand/Makefile

...racle.truffle.r.test/rpackages/rffi/testrffi/src/Makefile