Fix: mmap enum flags build failures

Some of the mmap option flags are not available on all architectures and
are defined to zero by include/linux/mman.h. This is probably done as a
way to no-op the use of these flags on configurations that don't support
them.
To fix this, only define these flags in our enumeration if they are
defined and non-zero.

Also, the MAP_HUGE_{2MB,1GB} labels were mistakingly named
MAP_HUGETLB_{2MB,1GB}.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I778a52a0da9da6e04231a52c7f68a22d122dfb83