BUILD: debug: mark the __start_mem_stats/__stop_mem_stats symbols as weak

BUILD: debug: mark the __start_mem_stats/__stop_mem_stats symbols as weak

Building with clang and DEBUG_MEM_STATS shows the following warnings:

  warning: __start_mem_stats changed binding to STB_WEAK [-Wsource-mgr]
  warning: __stop_mem_stats changed binding to STB_WEAK [-Wsource-mgr]

The reason is that the symbols are declared using ".globl" while they
are also referenced as __attribute__((weak)) elsewhere. It turns out
that a weak symbol is implicitly a global one and that the two classes
are exclusive, thus it may confuse the linker. Better fix this.

This may be backported where the patch applies.