build: turn on -fno-delete-null-pointer-checks

Work around spec violations in V8 where it checks that `this == NULL`.
GCC 6 started exploiting this particular kind of UB, resulting in
runtime crashes.

Fixes: https://github.com/nodejs/node/issues/6724
PR-URL: https://github.com/nodejs/node/pull/6738


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>