runtime: initialise cpu.HWCap on openbsd/arm64

OpenBSD does not provide auxv, however we still need to initialise cpu.HWCap.
For now initialise it to the bare minimum, until some form of CPU capability
detection is implemented or becomes available - see issue #31746.

Updates #31656

Change-Id: I68c3c069319fe60dc873f46def2a67c9f3d937d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/174129
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/auxv_none.go

@@ -7,6 +7,7 @@
 // +build !dragonfly
 // +build !freebsd
 // +build !netbsd
+// +build !openbsd !arm64
 // +build !solaris
 
 package runtime

src/runtime/os_openbsd_arm64.go

@@ -4,6 +4,10 @@
 
 package runtime
 
+import (
+	"internal/cpu"
+)
+
 //go:nosplit
 func cputicks() int64 {
 	// Currently cputicks() is used in blocking profiler and to seed runtime·fastrand().
@@ -11,3 +15,10 @@ func cputicks() int64 {
 	// TODO: need more entropy to better seed fastrand.
 	return nanotime()
 }
+
+func sysargs(argc int32, argv **byte) {
+	// OpenBSD does not have auxv, however we still need to initialise cpu.HWCaps.
+	// For now specify the bare minimum until we add some form of capabilities
+	// detection. See issue #31746.
+	cpu.HWCap = 1<<1 | 1<<0 // ASIMD, FP
+}