1.0.42.8: fix build on OpenBSD/i386 -current and upcoming 4.8 release

1.0.42.8: fix build on OpenBSD/i386 -current and upcoming 4.8 release

 * Patch by Josh Elsasser, lp#615489.

 * On OpenBSD/i386, the structure that stores the FPU state during
   signals was recently moved.

   Add a script to tools-for-build/ which is run from make-config.sh
   to detect where the FPU state is located, and create a
   src/runtime/openbsd-sigcontext.h header containing a #define which
   src/runtime/x86-bsd-os.c can use.

   This is necessary to build SBCL on recent -current snapshots of
   OpenBSD, and for the upcoming 4.8 release.

 * Missing NEWS entry for last commit.