Small refinement on the memory segment support in core files that

Greg added in r272276 -- when working with a non-user-process mach-o
core file, force the permissions to readable + executable, else the
unwinder can stop backtracing early if it gets a pc value in a segment
that it thinks is non-executable.
<rdar://problem/27138456> 
<rdar://problem/27462904> 

llvm-svn: 277065