src: add linux getauxval(AT_SECURE) in SafeGetenv

This commit attempts to fix the following TODO:
// TODO(bnoordhuis) Should perhaps also check whether
getauxval(AT_SECURE) is non-zero on Linux.

This can be manually tested at the moment using the following steps:

$ setcap cap_net_raw+ep out/Release/node
$ NODE_PENDING_DEPRECATION="1" out/Release/node -p
"process.binding('config').pendingDeprecation"
true
$ useradd test
$ su test
$ NODE_PENDING_DEPRECATION="1" out/Release/node -p
"process.binding('config').pendingDeprecation"
undefined

PR-URL: https://github.com/nodejs/node/pull/12548


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>