Skip gdb.base/libsegfault.exp on Windows

Skip gdb.base/libsegfault.exp on Windows

Running gdb.base/libsegfault.exp on Cygwin/MSYS2 causes badness.

E.g. on Cygwin:

 $ make check-parallel TESTS="gdb.base/libsegfault.exp"
 ...
 Running /home/alves/gdb/src/gdb/testsuite/gdb.base/libsegfault.exp ...
       0 [main] expect 6611 C:\cygwin64\bin\expect.exe: *** fatal error in forked process - error while loading shared libraries: libSegFault.so: cannot open shared object file: No such file or directory
				*** starting '"C:\cygwin64\bin\dumper.exe" -n "C:\cygwin64\bin\expect.exe" 8012' for pid 6611, tid 7992
 *** continuing pid 6611
 parent: sync byte write: broken pipe
 make[1]: *** [Makefile:320: check/gdb.base/libsegfault.exp] Error 255
 make[1]: Target 'do-check-parallel' not remade because of errors.
 make[1]: Leaving directory '/home/alves/gdb/build-cygwin-testsuite'
 outputs/gdb.base/libsegfault/gdb.sum: no recognised summary line
 outputs/gdb.base/libsegfault/gdb.log: no recognised summary line
 make: *** [Makefile:260: check-parallel] Error 2
 $

I.e., Expect's spawn fails, and that brings down the whole Expect
process.  The result is that in serial mode, the test run just stops.
And in parallel mode we end up with incomplete gdb.sum/gdb.log files,
which dg-extract-results.sh can't process.

Since libSegFault.so is a glibc feature that doesn't exist on
Cygwin/MSYS2, just skip the test there.

Approved-by: Kevin Buettner <kevinb@redhat.com>
Change-Id: I44bdbffa25a5d21c5019418b68776d334a57be26