Don't define _FORTIFY_SOURCE on MinGW

Recent MinGW versions require -lssp when using _FORTIFY_SOURCE, which
gdb does (in common-defs.h)
https://github.com/msys2/MINGW-packages/issues/5868#issuecomment-544107564

To avoid all the complications with checking for -lssp and making sure it's
linked statically, just don't define it.

gdb/ChangeLog:

2020-01-09  Christian Biesinger  <cbiesinger@google.com>

	* gdbsupport/common-defs.h: Don't define _FORTIFY_SOURCE on MinGW.

Change-Id: Ide6870ab57198219a2ef78bc675768a789ca2b1d