deps: no /safeseh for ml64.exe

`ml64.exe` doesn't support `/safeseh` option. Do not attempt to use it
if `target_arch=="x64"`.

See: https://msdn.microsoft.com/en-us/library/s0ksfwcf.aspx
PR-URL: https://github.com/nodejs/node/pull/7759


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>