ART: More ELF and oat file safety measures

In an ELF file, look for a shstrtab section when loading in
program-header-only mode. If the section is outside the file size,
it strongly indicates a broken compile.

When compiling oat files in the class linker, explicitly unlink
on failure. This should catch cases when dex2oat is killed or
crashes and doesn't have a chance to delete its (partial) output.

Bug: 15567083

(cherry picked from commit ad00fed9)

Change-Id: Ia0c75f151d91c6f26a71696967255d6d409ca882