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/mem/localalloc.h

@@ -287,27 +287,6 @@ namespace snmalloc
       return local_cache.remote_allocator;
     }
 
-    /**
-     * 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;
-    }
-
     /**
      * Call `SharedStateHandle::is_initialised()` if it is implemented,
      * unconditionally returns true otherwise.
@@ -639,9 +618,9 @@ namespace snmalloc
         if constexpr (location == Start)
           return start;
         else if constexpr (location == End)
-          return pointer_offset(start, rsize);
-        else
           return pointer_offset(start, rsize - 1);
+        else
+          return pointer_offset(start, rsize);
       }
 #else
       UNUSED(p_raw);
@@ -649,7 +628,7 @@ namespace snmalloc
 
       if constexpr ((location == End) || (location == OnePastEnd))
         // We don't know the End, so return MAX_PTR
-        return pointer_offset<void, void>(nullptr, UINTPTR_MAX);
+        return reinterpret_cast<void*>(UINTPTR_MAX);
       else
         // We don't know the Start, so return MIN_PTR
         return nullptr;