ProcessMachCore scans through the core file pages looking for a

user process dyld binary and/or a mach kernel binary image.  By
default, it prefers the kernel if it finds both.

But if it finds two kernel binary images (which can happen when
random things are mapped into memory), it may pick the wrong
kernel image.  

DynamicLoaderDarwinKernel has heuristics to find a kernel in memory;
once we've established that there is a kernel binary in memory,
call over to that class to see if it can find a kernel address via
its search methods.  If it does, use that.

Some minor cleanups to DynamicLoaderDarwinKernel while I was at it.

<rdar://problem/24446112> 

llvm-svn: 259983