examples: cpu feature checks (#171)

Add necessary feature checks to the dot and geohash examples to prevent illegal instruction errors.

Fixes #170 #153

examples/geohash/geohash_test.go

@@ -1,10 +1,17 @@
 package geohash
 
-import "testing"
+import (
+	"testing"
+
+	"golang.org/x/sys/cpu"
+)
 
 //go:generate go run asm.go -out geohash.s -stubs stub.go
 
 func TestEncodeIntMountEverest(t *testing.T) {
+	if !(cpu.X86.HasSSE2 && cpu.X86.HasBMI2) {
+		t.Skip("requires SSE2 and BMI2 instruction sets")
+	}
 	if EncodeInt(27.988056, 86.925278) != 0xceb7f254240fd612 {
 		t.Fail()
 	}