Change the kernel searching code to not go through the

memory cache subsystem so we're reading only the 4 bytes
needed to check for the magic word at the start of a mach-o
binary instead of the default 512 block.  It can be a small
performance help to reduce the size of memory reads from 
possibly unmapped memory.

<rdar://problem/29256385> 

llvm-svn: 286926