process: make Symbol.toStringTag writable

The ecosystem broke by making it non-writable, so this is a good intermediate fix.

Refs: https://github.com/nodejs/node/pull/25963#issuecomment-470456374

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines