[Driver][Gnu] -r: imply -nostdlib like GCC

See `gcc -dumpspecs` that -r essentially implies -nostdlib and suppresses
default -l* and crt*.o. The behavior makes sense because otherwise there will be
assuredly conflicting definitions when the relocatable output is linked into the
final executable/shared object.

Reviewed By: thesamesam, phosek

Differential Revision: https://reviews.llvm.org/D116843