There are some Linux binaries that expect the system to obey the "addr"
parameter to mmap(2), even if MAP_FIXED is not explicitly specified.
Android ART is one example. Implement bug compatibility for this case
in linuxulator.

Reviewed by: dchagin@
MFC after: 2 weeks
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D9373