Add vectorization sample.

This adds a new generic vectorization sample to replace hello-neon. Most
importantly, it covers non-Neon options for SIMD. One of the useful
things it shows, for example, is that there's actually no reason to
write SIMD code the way that hello-neon does any more.

This also solves a much simpler problem (small matrix multiplication),
which makes it easier to see how to deal with the SIMD features rather
than figuring out what a FIR filter is.

Finally, this sample benchmarks each of the implementations so it's
obvious what is and isn't worth doing. I was sort of surprised that
auto-vectorization didn't do better, and was pleased to learn that
there's no reason at all to write Neon intrinsics.

I'll delete hello-neon after this merges and I've fixed up the doc
links.

https://github.com/android/ndk-samples/issues/1011

vectorization/src/main/cpp/omp_simd.h

@@ -0,0 +1,55 @@
+/*
+ * Copyright (C) 2024 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <stdint.h>
+
+#include "matrix.h"
+
+namespace samples::vectorization {
+
+/**
+ * Multiplies two compatible matrices and returns the result.
+ *
+ * @tparam T The type of each matrix cell.
+ * @tparam M The number of rows in the left operand and the result.
+ * @tparam N The number of columns in the left operand, and the rows in the
+ * right operand.
+ * @tparam P The number of columns in the right operand and the result.
+ * @param lhs The left operand.
+ * @param rhs The right operand.
+ * @return The result of lhs * rhs.
+ */
+template <typename T, size_t M, size_t N, size_t P>
+Matrix<M, P, T> MultiplyWithOpenMP(const Matrix<M, N, T>& lhs,
+                                   const Matrix<N, P, T>& rhs) {
+  Matrix<M, P, T> result;
+#pragma omp simd
+  for (auto result_column_index = 0U; result_column_index < P;
+       result_column_index++) {
+    for (auto lhs_column_index = 0U; lhs_column_index < N; lhs_column_index++) {
+      auto lhs_column = lhs.column(lhs_column_index);
+      const T& scalar = rhs[lhs_column_index, result_column_index];
+      for (auto row = 0U; row < lhs_column.size(); row++) {
+        result[row, result_column_index] += lhs_column[row] * scalar;
+      }
+    }
+  }
+  return result;
+}
+
+}  // namespace samples::vectorization