ART: Make elf loading not abort

Changes elf_file code to use less CHECKs and instead return error
values (usually nullptr). This avoids aborts.

In oat_file, when loading an oat file fails, try to unlink at. If
this succeeds, on the next run we may compile again.

Bug: 17491333
Change-Id: I50fdd2edacd86f25d4dacf2180ce2a6105eaf4af