runtime: simplify openbsd check in usesLibcall and mStackIsSystemAllocated
The openbsd/mips64 runtime code was removed in CL 649659.
For #61546
Change-Id: I03f16c3396baddb0ee9aa751dd6f699a835e7586
Reviewed-on: https://go-review.googlesource.com/c/go/+/700976
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com> Reviewed-by: Michael Pratt <mpratt@google.com> Reviewed-by: Joel Sing <joel@sing.id.au> Reviewed-by: Keith Randall <khr@google.com> Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>