Check cpuflag for BMI2

Found out the hard way I shouldn't assume that having AVX doesn't imply
having BMI2.

Author: Jille

detect_avx2_always.go

@@ -9,3 +9,7 @@ func hasAVX() bool {
 func hasAVX2() bool {
 	return true
 }
+
+func hasBMI2() bool {
+	return true
+}

detect_avx2_never.go

@@ -9,3 +9,7 @@ func hasAVX() bool {
 func hasAVX2() bool {
 	return false
 }
+
+func hasBMI2() bool {
+	return false
+}

detect_avx2_runtime.go

@@ -11,3 +11,7 @@ func hasAVX() bool {
 func hasAVX2() bool {
 	return cpu.X86.HasAVX2
 }
+
+func hasBMI2() bool {
+	return cpu.X86.HasBMI2
+}

detect_avx2_testing.go

@@ -9,3 +9,7 @@ func hasAVX() bool {
 func hasAVX2() bool {
 	return false
 }
+
+func hasBMI2() bool {
+	return true
+}