runtime/runtime2: pack the sudog struct

This commit moves the isSelect bool below the ticket uint32.  The
boolean was consuming 8 bytes of the struct.  The uint32 was also
consuming 8 bytes, so we can pack isSelect below the uint32 and save 8
bytes.  This reduces the sudog struct from 96 bytes to 88 bytes.

Change-Id: If555cdaf2f5eaa125e2590fc4d113dbc99750738
GitHub-Last-Rev: d63b4e086b17da74e185046dfecb12d58e4f19ac
GitHub-Pull-Request: golang/go#36552
Reviewed-on: https://go-review.googlesource.com/c/go/+/214677


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>