Revert "runtime/cgo: use pthread_attr_get_np on Illumos"

Revert "runtime/cgo: use pthread_attr_get_np on Illumos"

This reverts CL 481795.

Reason for revert: CL 481061 causes C TSAN failures and must be
reverted. See CL 485275. This CL depends on CL 481061.

For #59678.

Change-Id: I5ec1f495154205ebdf19cd44c6e6452a7a3606f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/485315
Auto-Submit: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>