Eliminate 'condition sizeof(long)>4 is always true' cppcheck style warning

Eliminate 'condition sizeof(long)>4 is always true' cppcheck style warning

* src/atomic_ops_malloc.c (msb): Cast (s >> 32) to unsigned int type
before assigning the result to v; check __SIZEOF_SIZE_T__ value (if
defined) to decide whether (s >> 32) is needed; move condition
(sizeof(size_t) > 4) to SIZEOF_SIZE_T_GT_4 macro (to eliminate cppcheck
false report that the condition is always true).