Passing MAP_FIXED to mmap causes already mapped pages in the requested
region to be replaced. On Mac OS X this caused pages for JIT-generatd
code to appear in the memory range previously auto-allocated for the RAM
of the emulated machine. This led to a hang at boot time. The same problem
can probably occur on FreeBSD, but not on Linux since MAP_32BIT is used
there instead of MAP_FIXED.
The solution is to not use MAP_FIXED, but instead rely on the OS honoring
the hinted address which is below 4 GB: we don't need an exact match,
just a low address.