Add libc::memcpy with Checked/ReadsChecked template parameters

Expose snmalloc's checked memcpy through the libc API surface:
  libc::memcpy<Checked, ReadsChecked>(dst, src, len)

Checked controls destination bounds checking, ReadsChecked controls
source bounds checking (defaults to same as Checked).

Explicit instantiations provided in testlib for the common variants:
<true,true>, <true,false>, <false,false>.

Author: mjp41

src/snmalloc/global/libc.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "globalalloc.h"
+#include "memcpy.h"
 
 #include <errno.h>
 #include <string.h>
@@ -225,4 +226,16 @@ namespace snmalloc::libc
     *memptr = p;
     return 0;
   }
+
+  /**
+   * Checked memcpy through the libc API surface.
+   * @tparam Checked  check the destination is within a snmalloc allocation
+   * @tparam ReadsChecked  also check the source
+   */
+  template<bool Checked, bool ReadsChecked = Checked>
+  SNMALLOC_API void*
+  memcpy(void* dst, const void* src, size_t len)
+  {
+    return snmalloc::memcpy<Checked, ReadsChecked>(dst, src, len);
+  }
 } // namespace snmalloc::libc

src/test/snmalloc_testlib.cc

@@ -160,6 +160,14 @@ namespace snmalloc
     return MAX_SMALL_SIZECLASS_BITS;
   }
 
+  // -- Explicit instantiations of libc::memcpy variants --------------------
+  namespace libc
+  {
+    template void* memcpy<true, true>(void*, const void*, size_t);
+    template void* memcpy<true, false>(void*, const void*, size_t);
+    template void* memcpy<false, false>(void*, const void*, size_t);
+  } // namespace libc
+
   // -- PAL/AAL wrappers ----------------------------------------------------
 
   size_t pal_address_bits()

src/test/snmalloc_testlib.h

@@ -125,6 +125,9 @@ namespace snmalloc
     void* __malloc_start_pointer(void* ptr);
     void* __malloc_last_byte_pointer(void* ptr);
     void* __malloc_end_pointer(void* ptr);
+
+    template<bool Checked, bool ReadsChecked = Checked>
+    void* memcpy(void* dst, const void* src, size_t len);
   } // namespace libc
 
   /**