TSCLibc: fix `mkstemps` on Windows
TSCLibc: fix `mkstemps` on Windows

The original implementation of `mkstemps` that I had done was actually
not accounting for the template restriction.  Now that it is possible to
build packages with s-p-m to a certain extent, this is important to be
correct.  Account for the suffixing on the template.

apple/swift-tools-support-core,apple/swift-tools-support-core,apple/swift-tools-support-core,apple/swift-tools-support-core