src: merge `fn_name` in NODE_SET_PROTOTYPE_METHOD

Merge two duplicate `fn_name` into one in NODE_SET_PROTOTYPE_METHOD,
it may improve a bit performance.

PR-URL: https://github.com/nodejs/node/pull/13547


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>