Fix op/taint.t failure on Windows with 64-bit gcc-4.8.0 from MinGW-w64

Fix op/taint.t failure on Windows with 64-bit gcc-4.8.0 from MinGW-w64

MinGW-w64's 64-bit gcc now uses the SEH C++ exception model rather than
SJLJ or Dwarf2 (see http://mingw-w64.sourceforge.net/download.php).  For
such gccs, op/taint.t will need libgcc_s_seh-1.dll rather than
libgcc_s_sjlj-1.dll or libgcc_s_dw2-1.dll.  Fixes perl #121699.

Likewise, libwinpthread-1.dll is necessary if gcc was compiled with POSIX
threading and linked with the libwinpthread library, which is the case for
the gcc-4.8.2 toolchain that is likely to be used with the Strawberry Perl
5.20.x series.  Thanks to kmx for this.