When generating against the Win64 EH scheme, set the handler to the GCC-specific

When generating against the Win64 EH scheme, set the handler to the GCC-specific
handler.

At this moment, only GCC-style exceptions are supported. Other kinds
of exceptions, including "traditional" SEH and Microsoft Visual C++ exceptions,
need more work--and an compiler exception model that isn't specific to
GCC-style exceptions!

In particular, I imagine that it would be possible to mix "traditional" SEH
with GCC-style EH or Microsoft C++ EH. Currently LLVM has no way (beyond some
target-specific defaults and whole-module compiler switches) of knowing which
scheme to use when.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132283 91177308-0d34-0410-b5e6-96231b3b80d8