runtime: restore cgo_import_dynamic for libc.so on openbsd

This was removed in change 285692, however we need to explicitly pull libc.so
in when libpthread.so is being used. The current code works on openbsd/amd64
since we pull libc.so in via runtime/sys_openbsd2.go, however openbsd/arm64
does not do this currently.

Change-Id: Ibe93d936a22e69e2fe12620f6d27ccca7a91dba5
Reviewed-on: https://go-review.googlesource.com/c/go/+/285912


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>