FRV: Stop gcc from generating uninitialised variable warnings after BUG()

Stop gcc from generating uninitialised variable warnings after BUG().
The problem is that FRV's call into its gdbstub appears to return (if
the function is marked noreturn, then the compiler is under no
obligation to pass it a return address, and so GDB won't know where the
bug happened).

To get around this, we make the do...while wrapper in _debug_bug_trap()
an endless loop from which there's no escape.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>