brk emulation: remove MAP_FIXED for the initial allocation
brk emulation: remove MAP_FIXED for the initial allocation

In some cases the initial brk allocation was replacing existing mappings
because of the MAP_FIXED argument. By removing it, we allow the OS to
allocate at an alternative address.

TODO: Are any applications going to error out when the brk is not
in the expected location?

beehive-lab/mambo,beehive-lab/mambo