[ARM] Report fatal error for wrong ARM entry point
[ARM] Report fatal error for wrong ARM entry point

The case is possible with wrong input, so report
an error instead of using llvm_unreachable.


git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@233899 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld