Make munmap() actually work

[whitequark]

Before this commit, he header of a mapped area, struct map, was defined as follows:

struct map {
    int prot;
    int flags;
    off_t offset;
    size_t length;
    char body[];
};

Because the size and alignment of an off_t is 8 bytes, the entire structure was padded to 24 bytes. However, the offset of body into struct map was only 20 bytes. Therefore the code in mmap() and munmap() did not agree on the offset from header to body.

This commit changes mmap() to skip the entire header, which is what munmap() expects and what the size calculation uses.

For the curious, the code in mmap() that calculates the offset looks like:

(local.set $6
 (i32.add
  (local.get $6)
  (i32.const 20)))

and the code in munmap() that cancels it out for freeing looks like:

(call $fimport$5
 (i32.add
  (local.get $0)
  (i32.const -24)))

[sunfishcode]

Wow, thanks for tracking down this super subtle bug!