AArch64: Add memory allocation in low 4GB

AArch64: Add memory allocation in low 4GB

MAP_32BIT is implemented for only x86_64. Other 64bit platforms
don't have an equivalent.

This is a unsophisticated implemention of a scheme using msync and
MAP_FIXED to reproduce the functionality of MAP_32BIT.

Adds MAP_FIXED to RemapAtEnd, as the address used for the new mmap
doesn't get located correctly without it on aarch64.

Add MemMap::next_mem_pos_ to store last position. Add a safety assert.

Change-Id: I61871ff7fc327554c43e1d7f448c3d376490f1ea