Revert r75581: it causes massive breakage in the Ada

testsuite, due to exception handling not working
correctly.  Maybe because the libgcc unwinder is
miscompiled - not sure, and I won't have time to
look into it before leaving on holiday.  Note that
miscompilations of libgcc are not picked up by the
nightly testers, because they dynamically link with
libgcc, so pick up the system version rather than
the version built as part of llvm-gcc.  This is a
nasty flaw in the nightly testers.  (On the other
hand the Ada testsuite links with the just built
libgcc).

llvm-svn: 76895