Small further refinement to the check in ObjectFileMachO::ParseSymtab

which looks for binaries missing an LC_FUNCTION_STARTS section because
it was stripped/not emitted.  If we see a normal user process binary
(executable, dylib, framework, bundle) without LC_FUNCTION_STARTS, that
is unusual and we should disallow instruction emulation because that
binary has likely been stripped a lot.

If this is a non-user process binary -- a kernel, a standalone bare-board
binary, a kernel extension (kext) -- and there is no LC_FUNCTION_STARTS,
we should not assume anything about the binary and allow instruction
emulation as we would normally do.

<rdar://problem/26453952> 

llvm-svn: 270818