Handle ARM ELF symbols properly: skip $t* and $a* symbols in ObjectFileELF.

ELF objects contain marker symbols to differentiate between ARM and
THUMB functions. Instead of storing them internally and having garbage
show up when symbols are searched for by the user, we can just skip them
and not store them at all, as we never actually need them.

Change by Stephane Sezer.

Tested:
Ubuntu 14.04 x86_64
MacOSX 10.9.4 x86_64

llvm-svn: 217782