runtime: restore cgo_import_dynamic for libc.so on openbsd
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 <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@sing.id.au>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Run-TryBot: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,nawawi/go,nawawi/go,golang/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,golang/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go