Only enable ProcessElfCore if this is an ELF file.  Right now this plugin is winning over
ProcessMachCore because it doesn't check the format of the file up front.  (this is only now
happening because of Greg's change in r203178 to build ProcessElfCore for non-Linux/FreeBSD
hosts).
<rdar://problem/16257533> 


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@203205 91177308-0d34-0410-b5e6-96231b3b80d8