runtime: tricky replacements of _g_ in os3_solaris.go

The argument to semacreate is always getg().m.

Change-Id: I4c4104cd6b57ab6b9ed0b7d11b8089bcc9fdd621
Reviewed-on: https://go-review.googlesource.com/c/go/+/418583


Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>