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