Add a NetBSD PAL.

Currently untested, but identical to the FreeBSD one so should work...
The NetBSD man pages for `madvise` and `mmap` mention the flags that we
use.

src/pal/pal.h

@@ -13,6 +13,7 @@ namespace snmalloc
 #  include "pal_free_bsd_kernel.h"
 #  include "pal_freebsd.h"
 #  include "pal_linux.h"
+#  include "pal_netbsd.h"
 #  include "pal_openbsd.h"
 #  include "pal_windows.h"
 #endif
@@ -33,6 +34,8 @@ namespace snmalloc
     PALFreeBSDKernel;
 #  elif defined(__FreeBSD__)
     PALFBSD;
+#  elif defined(__NetBSD__)
+    PALNetBSD;
 #  elif defined(__OpenBSD__)
     PALOBSD;
 #  else

src/pal/pal_netbsd.h

@@ -0,0 +1,29 @@
+#pragma once
+
+#ifdef __NetBSD__
+#  include "pal_bsd_aligned.h"
+
+namespace snmalloc
+{
+  /**
+   * NetBSD-specific platform abstraction layer.
+   *
+   * This adds NetBSD-specific aligned allocation to the generic BSD
+   * implementation.
+   */
+  class PALNetBSD : public PALBSD_Aligned<PALFBSD>
+  {
+  public:
+    /**
+     * Bitmap of PalFeatures flags indicating the optional features that this
+     * PAL supports.
+     *
+     * The NetBSD PAL does not currently add any features beyond those of a
+     * generic BSD with support for arbitrary alignment from `mmap`.  This
+     * field is declared explicitly to remind anyone modifying this class to
+     * add new features that they should add any required feature flags.
+     */
+    static constexpr uint64_t pal_features = PALBSD_Aligned::pal_features;
+  };
+} // namespace snmalloc
+#endif