Use the cmake variable `WAMR_BUILD_GLOBAL_HEAP_POOL` and
`WAMR_BUILD_GLOBAL_HEAP_SIZE` to enable/disable the global heap pool
and set its size. And set the default global heap size in core/config.h and
the cmake files.
As a result, the developers who build iwasm can easily enable/disable the
global heap pool and change its size regardless of the iwasm implementation,
without manually finding and patching the right location for that value.
