Only one person committed with these email addresses, so there's no need to use
the map-different-names-for-one-email-address syntax.

No behavior change.

Differential Revision: https://reviews.llvm.org/D108007

The bot did its first 43 commits under the name "GN Syncbot" before
it switched to "LLVM GN Syncbot". Use the latter as canonical name.

Add aliases for various alternative email addresses.

I'm using rnk@google.com as the canonical email for now, but I guess I
can always update .mailmap in the future.

See "Proposal: Adding a toplevel .mailmap file" on llvm-dev:
https://lists.llvm.org/pipermail/llvm-dev/2021-May/150741.html

Differential Revision: https://reviews.llvm.org/D103360