Add support for dwarf unwinding on windows on x86_64

Clang doesn't currently support building for windows/x86_64 with
dwarf by setting command line parameters, but if manually modified
to use dwarf, we can make libunwind work in this configuration
as well.

Also include i386 in the docs when adding this as a supported
configuration; libunwind already works for i386 windows, but
can fail due to an issue unrelated to windows itself.

Differential Revision: https://reviews.llvm.org/D38819

llvm-svn: 316747