sync: Enable the fast path of select()

This was erroneously disabled as part of 065e121f and it hasn't been turned on
since. This was a 3x perf improvement in a test of mine.
auto merge of #15044 : alexcrichton/rust/speed-up-select, r=kballard

This was erroneously disabled as part of 065e121f and it hasn't been turned on
since. This was a 3x perf improvement in a test of mine.