[Driver] Make -print-libgcc-file-name print compiler-rt lib when used

Make the -print-libgcc-file-name option print an appropriate compiler
runtime library, that is libgcc.a if gcc runtime is used
and an appropriate compiler-rt library if that runtime is used.

The main use for this is to allow linking executables built with
-nodefaultlibs (e.g. to avoid linking to the standard C++ library) to
the compiler runtime library, e.g. using:

  clang++ ... -nodefaultlibs $(clang++ ... -print-libgcc-file-name)

in which case currently a program built like this linked to the gcc
runtime unconditionally. The patch fixes it to use compiler-rt libraries
instead when compiler-rt is the active runtime.

Differential Revision: https://reviews.llvm.org/D25338

llvm-svn: 283572