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.
Various fixes and beautifications coordinated with @1c3t3a,
fixes 2 of the 3 all remaining issues from #892:
- enable to os_mmap executable memory
- fix os_malloc/os_realloc/os_free issues
- implement os_thread_get_stack_boundary
- add build scripts to include with esp-idf to use wamr as
an ESP-IDF component
- update sample and document
