CPU dispatching: additional AVX-512 check for mingw-w64

[mshabunin]

This pullrequest changes

Problem:

[ 58%] Building CXX object modules/dnn/CMakeFiles/opencv_dnn.dir/layers/layers_common.avx512_skx.cpp.obj
%USERPROFILE%\AppData\Local\Temp\ccBSUeSO.s: Assembler messages:
%USERPROFILE%\AppData\Local\Temp\ccBSUeSO.s:21573: Error: invalid register for .seh_savexmm
%USERPROFILE%\AppData\Local\Temp\ccBSUeSO.s:21575: Error: invalid register for .seh_savexmm
%USERPROFILE%\AppData\Local\Temp\ccBSUeSO.s:21577: Error: invalid register for .seh_savexmm
<...>
mingw32-make[2]: * [modules\dnn\CMakeFiles\opencv_dnn.dir\build.make:1742: modules/dnn/CMakeFiles/opencv_dnn.dir/layers/layers_common.avx512_skx.cpp.obj] Error 1 
mingw32-make[1]: [CMakeFiles\Makefile2:4024: modules/dnn/CMakeFiles/opencv_dnn.dir/all] Error 2 
mingw32-make: ** [Makefile:162: all] Error 2

Proposed patch is a workaround based on this patch in GCC: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79127 - AVX-512 support will not be detected by cmake for mingw-w64, and will be disabled.

docker_image:Custom=ubuntu:17.10

[alalek]

Related: https://sourceforge.net/p/mingw-w64/bugs/681/
There is suggestion to use: "-fno-asynchronous-unwind-tables' compiler option