Resolve issues for unit tests on 32 bits

[qinsoon]

This PR resolves a few issues we recently saw for running unit tests in our CI.

• Fix an issue in the tests for byte map mmapper. In the clean up step, we do munmap for a larger area than what we actually mmapped in the test. This caused an issue that we may munmap regions that are used by other code. Segfault in unit tests on 32bits #1092 is caused by this -- the munmap after the tests unmaps the memory used by Rust, and caused Rust to segfault after the tests.
• Change the test address for mmap tests to reduce the chance that we may fail in mmap in the tests.
• Make raw_memory_freelist only compiled in 64bits. This mitigates the issue we saw in RawMemoryFreeList can't get more space from mmap #1091 for raw_memory_freelist tests.

[wks]

Inside the with_cleanup( || { ... }) block, it computes the number of pages to mmap separately (the expression pages_per_chunk * 2). We have to manually make sure this size is the same as MMAP_CHUNK_BYTES * 2 which is the definition of test_memory_size.

It is better if we can hoist the computations before the invocation of with_cleanup, like how you changed other test cases, and compute the number of pages to mmap and mprotect from the test_memory_size. That will make the two values dependent on each other, and make the test case more robust. For example:

let test_memory_bytes = MMAP_CHUNK_BYTES * 2;
let protect_memory_bytes = test_memory_bytes / 2; // Now this depends on test_memory_bytes
let test_memory_pages = test_memory_bytes >> LOG_BYTES_IN_PAGE as usize; // This also depends on test_memory_bytes
let protect_memory_pages = protect_memory_bytes >> LOG_BYTES_IN_PAGE as usize; // This also depends on test_memory_bytes
with_cleanup(|| {
    // ...
    mmapper.ensure_mapped(FIXED_ADDRESS, test_memory_pages).unwrap();
    mmapper.protect(FIXED_ADDRESS, protect_memory_pages);
    // ...
}, || {
    memory::munmap(FIXED_ADDRESS, test_memory_pages).unwrap();
})

[qinsoon]

Done

[wks]

LGTM