A little cleanup / commenting on locating kernel binaries while I

was working on something else.
DynamicLoaderDarwinKernel::SearchForKernelNearPC should have had
an early return if the pc value is not in high memory; add that.
The search for a kernel at 0x2000 offsets was a stopgap; it doesn't
need to be checked any longer.

llvm-svn: 350786