um: Make stack trace reliable against kernel mode faults

As UML uses an alternative signal stack we cannot use
the current stack pointer for stack dumping if UML itself
dies by SIGSEGV. To bypass this issue we save regs taken
from mcontext in our segv handler into thread_struct and
use these regs to obtain the stack pointer in show_stack().

Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/include/shared/os.h

@@ -227,6 +227,7 @@ extern void block_signals(void);
 extern void unblock_signals(void);
 extern int get_signals(void);
 extern int set_signals(int enable);
+extern int os_is_signal_stack(void);
 
 /* util.c */
 extern void stack_protections(unsigned long address);