Since the signal trampoline is now execute-only we no longer write it

Since the signal trampoline is now execute-only we no longer write it
into core dumps.  As a result backtraces through signal handlers no
longer work in gdb and other debuggers.

Fix this by keeping a read-only mapping of the signal trampoline in the
kernel and writing it into the core dump at the virtual address where it
is mapped in the process.

ok deraadt@, tb@