ART: Do not mmap boot image tables for non-executable oat files.

Fix zygote crash causing boot loop in fall-back mode (trying
to run without boot image) by allowing loading of oat files
that need mmapped boot image tables as non-executable
without the boot image.

No regression test provided (would require boot test).

Test: Rely on TreeHugger.
Bug: 74092284
Change-Id: Ic233ac470a2ae0151235f9b2145c4aa529348b3a