runtime: don't overallocate in select code
runtime: don't overallocate in select code

If we've already allocated an fd_set, don't allocate another one.

Also, don't bother to read from rdwake if it wasn't returned in select.

Fixes https://gcc.gnu.org/PR61303.

Change-Id: I20ef87c7e983a0ef49d2546c8f2ac9c7b6ed97a9
Reviewed-on: https://go-review.googlesource.com/17243
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend