In current gdb.base/savedregs.exp, signal handler is installed for
signal SIGSEGV, and SIGSEGV is trigger by `*(char *)0 = 0;'.  However,
on non-mmu uclinux system, writing to an address 0x0 doesn't trigger
SIGSEGV.

In my patch, SIGILL is chosen to replace SIGSEGV.  One assumption here
is that 0xffff is an invalid instruction on all ports.

FAILs go away on my non-mmu uclinux system, and no regression on
i686-pc-linux-gnu/x86_64-unknown-linux-gnu/armv7l-unknown-linux-gnueabi.
 OK to apply?

-- 
Yao (齐尧)