internal/cpu: fix arm64 build

I submitted two CLs which broke the build. Add temporary placeholder
with false bools to fix the build and restore old behavior.

Updates golang/go#22718 (details of why it broke)

Change-Id: I1f30624e14f631a95f4eff5aae462f1091f723a2
Reviewed-on: https://go-review.googlesource.com/77590Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/internal/cpu/cpu_arm64.go

@@ -5,3 +5,10 @@
 package cpu
 
 const CacheLineSize = 64
+
+// TODO: delete this once https://go-review.googlesource.com/c/go/+/76490 lands.
+// These will just be false for now.
+var ARM64 struct {
+	HasSHA1 bool
+	HasSHA2 bool
+}