hurd: unwinding support over signal trampolines

hurd: unwinding support over signal trampolines

This allows to get full backtrace from signal handlers, otherwise the
backtrace stops at the trampoline that calls the handler.

This needs special knowledge how the trampoline records register context
for the sigreturn call after signal handling.

gdb/ChangeLog:

	* i386-gnu-tdep.c: Include "gdbcore.h"
	(gnu_sigtramp_code, i386_gnu_sc_reg_offset): New arrays.
        (GNU_SIGTRAMP_LEN, GNU_SIGTRAMP_TAIL,
        I386_GNU_SIGCONTEXT_THREAD_STATE_OFFSET): New macros
        (i386_gnu_sigtramp_start, i386_gnu_sigtramp_p,
        i386_gnu_sigcontext_addr): New functions
        (i386gnu_init_abi): Register i386_gnu_sigtramp_p,
        i386_gnu_sigcontext_addr, and i386_gnu_sc_reg_offset in the gdbarch
        tdep.