bfin: fix fp reference in _JMPBUF_UNWINDS

bfin: fix fp reference in _JMPBUF_UNWINDS

We want to access the frame pointer, so do so directly rather than
"overflowing" the pregs array and ending up at the fp member.

This fixes the Blackfin build warnings:
libpthread/linuxthreads.old/ptlongjmp.c: In function 'pthread_cleanup_upto':
libpthread/linuxthreads.old/ptlongjmp.c:35: warning: array subscript is above array bounds
libpthread/linuxthreads.old/ptlongjmp.c:56: warning: array subscript is above array bounds

Signed-off-by: Mike Frysinger <vapier@gentoo.org>