Add memcpy with bounds checks.

The memcpy implementation is not completely stupid but is almost
certainly not as good as a carefully tuned and optimised one.

Building snmalloc with FreeBSD's libc memcpy + jemalloc and with this,
each 10 times, does not show a statistically significant performance
difference at 95% confidence.  The snmalloc version has very slightly
lower median and worst-case times.  This is in no way a sensible
benchmark, but it serves as a smoke test for significant performance
regressions.

The CI self-host job now uses the checked memcpy.

This also fixes an off-by-one error in the external bounds.  This is
triggered by ninja, so we will see breakage in CI if it is reintroduced.

In debug builds, we provide a verbose error containing the address of
the allocation, the base and bounds of the allocation, and a backtrace.

The backtrace was broken by the CI cleanup moving the BACKTRACE_HEADER
macro into the SNMALLOC_ namespace.  This is also fixed.

The test involves hijacking `abort`, which doesn't work everywhere.  It
also requires `backtrace` to work in configurations where stack traces
are enabled.  This is disabled in QEMU because `backtrace` appears to
crash reliably in QEMU user mode.

For now, in the -checks build configurations, we are hitting a slow path
in the pagemap on accesses so that the pages that are `PROT_NONE` don't
cause crashes.  These need to be made read-only, but this requires a PAL
change.

src/backend/commonconfig.h

@@ -115,4 +115,26 @@ namespace snmalloc
       &unused_remote != fake_large_remote,
       "Compilation should ensure these are different");
   };
+
+  /**
+   * SFINAE helper.  Matched only if `T` implements `is_initialised`.  Calls
+   * it if it exists.
+   */
+  template<typename T>
+  SNMALLOC_FAST_PATH auto call_is_initialised(T*, int)
+    -> decltype(T::is_initialised())
+  {
+    return T::is_initialised();
+  }
+
+  /**
+   * SFINAE helper.  Matched only if `T` does not implement `is_initialised`.
+   * Unconditionally returns true if invoked.
+   */
+  template<typename T>
+  SNMALLOC_FAST_PATH auto call_is_initialised(T*, long)
+  {
+    return true;
+  }
+
 } // namespace snmalloc

src/backend/pagemap.h

@@ -244,7 +244,14 @@ namespace snmalloc
       }
 
       //  This means external pointer on Windows will be slow.
-      if constexpr (potentially_out_of_range && !pal_supports<LazyCommit, PAL>)
+      // TODO: With SNMALLOC_CHECK_CLIENT we should map that region read-only,
+      // not no-access, but this requires a PAL change.
+      if constexpr (
+        potentially_out_of_range
+#ifndef SNMALLOC_CHECK_CLIENT
+        && !pal_supports<LazyCommit, PAL>
+#endif
+      )
       {
         register_range(p, 1);
       }