Use FlatPageMap on OS with lazy commit

If the operating system will allocate private pages on demand for the
pagemap then use the FlatPageMap by default as it generates better code
for deallocation.

src/pal/pal_linux.h

@@ -18,7 +18,7 @@ namespace snmalloc
      * Bitmap of PalFeatures flags indicating the optional features that this
      * PAL supports.
      */
-    static constexpr uint64_t pal_features = 0;
+    static constexpr uint64_t pal_features = LazyCommit;
     static void error(const char* const str)
     {
       puts(str);