[v12.x backport] util: use a global symbol for `util.promisify.custom`
requested to merge github/fork/ExE-Boss/backport/v12.x/lib/util/use-global-util-promisify-custom-symbol into v12.x-staging
This backports https://github.com/nodejs/node/pull/31672 to Node v12.
It has already been backported to Node v13 as https://github.com/nodejs/node/commit/975d6b01447e426847baf9011a1c4ce15342fd6c.
Checklist
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make -j4 test(UNIX), orvcbuild test(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines