Retry mmap if it fails

Sometimes, mmap returns ENOMEM even though there is enough memory, in
that case, retried mmap mostly succeeds.

So as a workaround, let MemMapArena::Allocate retry at most 3 time.
(to resolve flaky build failure)

Bug: 278665389
Test: run dex2oat repeatedly
Change-Id: Icbde6a06fe8d1b189486ea2ab10003425248df52