bc13101cf9
Some signal handlers were set up within an !_MSC_VER condition,
i.e. omitted in MSVC builds but included in mingw builds. Previously
sigtstp_handler was defined in all builds, but since
4bcadd6686 / D120320 it's only
defined non platforms other than Windows.
Change the condition to !_WIN32 for consistency between the MSVC
and mingw builds, fixing the build for mingw.
Differential Revision: https://reviews.llvm.org/D122486