[Driver] Explicitly request platform rtlib in the Driver pic test

This test checks whether crtbegin.o and crtend.o appear on the link
line, but names of these files may be affected by the choice of the
rtlib, specifically when compiler-rt is used as the default rtlib
the names will be clang_rt.crtbegin.o and clang_rt.crtend.o instead
of crtbeginS.o and crtendS.o. To avoid the test failure, explicitly
request to use the platform rtlib.

Differential Revision: https://reviews.llvm.org/D61383

llvm-svn: 359706