build: enable BIND_NOW when compiling Linux builds, fix #15149. (#15153)

We've hardened Linux builds by enabling PIE and RELRO,
and should continue to try hardening Linux builds by
enabling BIND_NOW. With both RELRO and BIND_NOW enabled,
we can stop all GOT overwrite attacks. The same hardening
option has been enabled in official Chrome/Chromium
builds since more than five years ago.

This helps to improve the security of a whole range of
applications built upon Electron, including sensetive ones
such as Signal-Desktop.

Signed-off-by: Tom Li <tomli@tomli.me>

electron.gyp

@@ -257,6 +257,8 @@
             'ldflags': [
               # Build as Position-Independent Executable to mitigate exploitations.
               '-pie',
+              # Enable BIND_NOW to prevent GOT overwrite attacks.
+              '-Wl,-z,now',
             ],
           },
           'cflags_cc': [