runtime: don't crash holding locks on racy timer access

runtime: don't crash holding locks on racy timer access

If we run into data corruption due to the program accessing timers in
a racy way, do a normal panic rather than a hard crash with "panic
holding locks". The hope is to make the problem less confusing for users.

Fixes #25686

Change-Id: I863417adf21f7f8c088675b67a3acf49a0cdef41
Reviewed-on: https://go-review.googlesource.com/115815
Reviewed-by: Austin Clements <austin@google.com>