build: add '-z relro -z now' linker flags

These flags could make some sections and the GOT entries of node
process read only to avoid being modified after dynamic linking is
done, thus the security could be enhanced.

Fixes: https://github.com/nodejs/node/issues/20122
PR-URL: https://github.com/nodejs/node/pull/20513
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

node.gypi

@@ -291,7 +291,10 @@
     [ 'OS=="sunos"', {
       'ldflags': [ '-Wl,-M,/usr/lib/ld/map.noexstk' ],
     }],
-
+    [ 'OS in "freebsd linux"', {
+      'ldflags': [ '-Wl,-z,relro',
+                   '-Wl,-z,now' ]
+    }],
     [ 'node_use_openssl=="true"', {
       'defines': [ 'HAVE_OPENSSL=1' ],
       'conditions': [