__mh_execute_header atoms should be global and never dead strippped.

In r259574 I fixed some of the issues with the mach header symbols
and DSO handles.

This is the next issue whereby the __mh_execute_header has to not
be dead stripped, and (to match ld64) should be dynamically referenced.

The test here should also have been added in r259574 to make sure that
we emit this symbol.  But checking that it is not only emitted but also
has the correct reference type is fine.

llvm-svn: 259589