Race on sqlite3_open_v2/sqlite3_initialize on darwin/amd64

I received a report on zombiezen.com/go/sqlite of a data race when opening multiple connections to :memory: with shared-cache mode. The race is inside modernc.org/sqlite/lib.Xsqlite3_initialize inside a critical region protected by a call to Xsqlite3_mutex_enter. I'm not able to reproduce the issue on linux/amd64, and based on stack trace, Peter is running on darwin/amd64. I believe the root issue is that pthreads are only used on linux/amd64.

I couldn't find another issue tracking implementing mutex support on more than linux/amd64, so I'm opening this ticket.