deps: V8: use ATOMIC_VAR_INIT instead of std::atomic_init

`std::atomic_init<size_t>` is not implemented on all platforms.

PR-URL: https://github.com/nodejs/node/pull/27375


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Ujjwal Sharma <usharma1998@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>