temporary fix for [perl #121975] COW speedup lost after e8c6a474

Disable use of Perl_safesysmalloc_size by default. Only use it when PERL_USE_MALLOC_SIZE is defined.

Using Perl_safesysmalloc_size() perl cannot tell a deliberately preallocated buffer
which we probably dont want to COW from a malloc() preallocated buffer where we probably
dont care. Hopefully this fixes the slowdown observed on some platforms.