Add a new flag to allow a user to specify the initial heap size for init_emulated_brk().

The current initial heap size for init_emulated_brk() is 4M. emulate_app_brk() uses mremap to increase or decrease the size as needed.

There are applications, like SPEC 2017 gcc subrun 3 and 4 (uses 1.2G+), which increase the heap size through brk() . When emulate_app_brk() can't increase the size, gcc uses mmap to allocate new memory. This changes the behavior of the application.

Adding a new flag allows for a bigger initial heap size. If the heap size is reduced by the application, there is no guarantee the heap will be able to expand to the initial heap size or beyond.

I've tested an additional change to not shrink the heap size below the initial size to make sure the memory will be available. But gcc subrun 3 returns an internal error if the memory is not shrank when it uses brk to reduce the heap size.