cmake/target.cmake defines macros for the supported platforms, this
commit changes predefined system macros to our own macros.
__linux__ --> OS_LINUX
__APPLE__ --> OS_DARWIN
__FreeBSD__ --> OS_FREEBSD
There are lots of places where HashTable is used with
AllocatorWithStackMemory, but the size of allocator inline memory is set
incorrectly, and it's less than the initial HashTable buffer size.
Because of this, HashTable is always allocated on the heap, and the
inline memory becomes a useless dead weight.
For PODArray, we have previously added a helper template that makes sure
these values are in sync, so there was only one such discrepancy left,
in the unit test.
