Turns out GAS does have Win64 EH directives. (It also supports WinCE EH.) Make

Turns out GAS does have Win64 EH directives. (It also supports WinCE EH.) Make
ours compatible with GAS.

In retrospect, I should have emailed binutils about this earlier. Thanks to
Kai Tietz for pointing out that GAS already had SEH directives.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@131652 91177308-0d34-0410-b5e6-96231b3b80d8