z/OS currently allows allocation of pages of 1MB alignment
above the bar. This is a problem for the v8 heap space
allocator because most objects are aligned at smaller
boundaries. Inevitably this leads to below the bar memory
being exhausted and any further request results in an
OOM error.

For now we will force allocation above the bar even when
the alignment is below 1MB once we have run out of
memory below the bar. Eventually we will need to implement
a memory pool manager or wait till z/OS has anonymous mmap
support.

Fix an issue introduced in commit 12d693d
where reservation_ field may not be initalized with the result of a successful
anon_mmap() call.  In the deconstructor, the memory is released based on
reservation_ value.

Signed-off-by: Joran Siu <joransiu@ca.ibm.com>