runtime/cgo: use PTHREAD_{MUTEX,COND}_INITIALIZER

runtime/cgo: use PTHREAD_{MUTEX,COND}_INITIALIZER

Technically you must initialize static pthread_mutex_t and
pthread_cond_t variables with the appropriate INITIALIZER macro.  In
practice the default initializers are zero anyhow, but it's still good
code hygiene.

Change-Id: I517304b16c2c7943b3880855c1b47a9a506b4bdf
Reviewed-on: https://go-review.googlesource.com/9433
Reviewed-by: David Crawshaw <crawshaw@golang.org>