build: don't squash signal handlers with --shared

An application using node built as a shared library may legitimately
implement its own signal handling routines. Current behaviour is
to squash all signal handlers on node startup. This change will
stop that behaviour when node is built as a shared library.

PR-URL: https://github.com/nodejs/node/pull/10539
Fixes: https://github.com/nodejs/node/issues/10520
Refs: https://github.com/nodejs/node/pull/615


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>