Change ProcessElfCore::CreateInstance() to check if the ELF magic bytes
appear in the file before it assumes the file is ELF.  It was incorrectly
activating for Mach-O core files.
<rdar://problem/16836816> 


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@208265 91177308-0d34-0410-b5e6-96231b3b80d8