[NFC] Separate out support for aligned allocation from the FreeBSD PAL.

NetBSD also supports `MAP_ALIGNED()` in `mmap` (according to the man
page, at least).

src/pal/pal_bsd_aligned.h

@@ -0,0 +1,55 @@
+#pragma once
+
+#if defined(__FreeBSD__) && !defined(_KERNEL)
+#  include "pal_bsd.h"
+
+namespace snmalloc
+{
+  /**
+   * FreeBSD-specific platform abstraction layer.
+   *
+   * This adds aligned allocation using `MAP_ALIGNED` to the generic BSD
+   * implementation.  This flag is supported by NetBSD and FreeBSD.
+   */
+  template<class OS>
+  class PALBSD_Aligned : public PALBSD<OS>
+  {
+  public:
+    /**
+     * Bitmap of PalFeatures flags indicating the optional features that this
+     * PAL supports.
+     *
+     * This class adds support for aligned allocation.
+     */
+    static constexpr uint64_t pal_features =
+      AlignedAllocation | PALBSD<OS>::pal_features;
+
+    /**
+     * Reserve memory at a specific alignment.
+     */
+    template<bool committed>
+    void* reserve(const size_t* size, size_t align) noexcept
+    {
+      // Alignment must be a power of 2.
+      assert(align == bits::next_pow2(align));
+
+      align = bits::max<size_t>(4096, align);
+
+      size_t log2align = bits::next_pow2_bits(align);
+
+      void* p = mmap(
+        nullptr,
+        *size,
+        PROT_READ | PROT_WRITE,
+        MAP_PRIVATE | MAP_ANONYMOUS | MAP_ALIGNED(log2align),
+        -1,
+        0);
+
+      if (p == MAP_FAILED)
+        error("Out of memory");
+
+      return p;
+    }
+  };
+} // namespace snmalloc
+#endif