kgdb,i386: use address that SP register points to in the exception frame

The treatment of the SP register is different on x86_64 and i386.
This is a regression fix that lived outside the mainline kernel from
2.6.27 to now.  The regression was a result of the original merge
consolidation of the i386 and x86_64 archs to x86.

The incorrectly reported SP on i386 prevented stack tracebacks from
working correctly in gdb.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>