syntax: Parse GT tokens from `>=` and `>>=`

The parser already has special logic for parsing `>` tokens from `>>`, and this
commit extends the logic to the acquiring a `>` from the `>=` and `>>=` tokens
as well.

Closes #15043
auto merge of #15056 : alexcrichton/rust/issue-15043, r=kballard

The parser already has special logic for parsing `>` tokens from `>>`, and this
commit extends the logic to the acquiring a `>` from the `>=` and `>>=` tokens
as well.

Closes #15043