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
4bcadd66864bf4af929ac8753a51d7ad408cdef0 / 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
bc13101cf9