Fall back to true anonymous mmap if out of file descriptors.

MemMap::MapAnonymous needs a file descriptor to create an ashmem
region to label otherwise anonymous mmaps for debugging purposes. If
the process has no file descriptors available, fall back to
traditional anonymous mmap rather than failing.

Test: m test-art-host, m test-art-target
Bug: 32013594
Bug: 32302133
Change-Id: I6b2c770fc031eb8d429407f3a0e7408c52cb1985