Driver: use GNU::Link for the Generic_GCC toolchain

This changes the behaviour of the driver for linking to match that of the
Generic_GCC::Assemble.    The default link should use "ld" rather than "gcc" for
the linker as gcc does.  This avoids the unnecessary round-tripping through gcc.
It also is much more reasonable behaviour from the user's perspective.  This
should have been updated with SVN r195554 which changed the behaviour of
Generic_GCC::Assemble.

The gcc_forward test needs to be updated to mark the fact that -march is a flag
for GCC not ld.  This was updated as a typo fix, but added a check for a flag
that is not a link flag.

The bindings test covers the change for testing, and thus no new test was added.

llvm-svn: 211866