[build] Disable dangling pointer checking for GCC

[build] Disable dangling pointer checking for GCC

The dangling pointer warning introduced in GCC 12 reports false
positives that result in build failures.  In particular, storing the
address of a local code label used to record the current state of a
state machine (as done in crypto/deflate.c) is reported as an error.

There seems to be no way to mark the pointer type as being permitted
to hold such a value, so unconditionally disable the warning.

Signed-off-by: Michael Brown <mcb30@ipxe.org>