Raise stack size to 8MB for pthreads

It seems there is no other way to specify stack size on std::thread than linker
flags and the effective flags are named differently in many toolchains. On
toolchains where pthread is always available, this patch changes the stack
size change in our C++ code via pthread to ensure a minimum stack size of 8MB,
instead of relying on linker defaults which may be platform-specific.

Also raises default stack size on OSX to current Linux default (8MB) just to
be safe.

Closes https://github.com/official-stockfish/Stockfish/pull/2303

No functional change