Allow death test child to bypass WER under MinGW

The mechanics of suppressing debugger trapping and Windows Error Reporting for the crashed child process in a death test are currently guarded under the GTEST_HAS_SEH macro. This seems unnecessary, as the logic does not call any APIs related to Structured Error Handling.

Replace the guarding macro with the more permissive GTEST_OS_WINDOWS, so that Windows toolchains without SEH support (e.g. MinGW) can benefit from it.

Fixes: #1116