* score/cpu/sparc/cpu.c, score/cpu/sparc/rtems/score/cpu.h,
score/include/rtems/score/context.h, score/src/threadhandler.c: Fix
stack so gdb backtrace does not print corrupted frame message after
_Thread_Handler. Daniel Hellstrom <daniel@gaisler.com> provided the
SPARC implementation and I made it more general.