Fix parsing of the plt section for android-arm

The ELF data contains two different errors in some ELF files on android.
* The link field of the symbol table don't point to the plt section or
  to the dynsym section even when it is present in the ELF files.
* The size of the plt entries aren't specified in the section header of
  the plt section.

This CL adds some workarounds for these two issue with finding the
sections by name if the link field is empty and by using a heuristic to
calculate the size and offset of the plt entries.

Differential revision: http://reviews.llvm.org/D9560

llvm-svn: 236818