[lldb] Fix building for mingw after changes to sigtstp_handler

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
4bcadd66 / 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