Add GNU extensions mempcpy and wmemcpy.

Used by elfutils. On the bright side, they stopped using __mempcpy.

Bug: 18374026
Change-Id: Id29bbe6ef1c5ed5a171bb6c32182f129d8332abb

tests/string_test.cpp

@@ -1395,3 +1395,8 @@ TEST(string, strnlen_147048) {
   EXPECT_EQ(0U, strnlen(heap_src, 1024*1024*1024));
   delete[] heap_src;
 }
+
+TEST(string, mempcpy) {
+  char dst[6];
+  ASSERT_EQ(&dst[4], reinterpret_cast<char*>(mempcpy(dst, "hello", 4)));
+}