runtime: let freezetheworld work even when gomaxprocs=1

Freezetheworld still has stuff to do when gomaxprocs=1.
In particular, signals can come in on other Ms (like the GC M, say)
and the single user M is still running.

Fixes #10546

Change-Id: I2f07f17d1c81e93cf905df2cb087112d436ca7e7
Reviewed-on: https://go-review.googlesource.com/9551


Reviewed-by: Dmitry Vyukov <dvyukov@google.com>