runtime: correct syscall10/syscall10X on openbsd/amd64

The syscall10/syscall10X implementation uses an incorrect stack offset for
arguments a7 to a10. Correct this so that the syscall arguments work as
intended.

Updates #36435
Fixes #43927

Change-Id: Ia7ae6cc8c89f50acfd951c0f271f3b3309934499
Reviewed-on: https://go-review.googlesource.com/c/go/+/287252


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Zhang <cherryyz@google.com>