gfortran MAIN__ symbol not marked TREE_PUBLIC

[David Edelsohn]

	With Paul's change to gcc/fortran/symbol.c:gfc_clear_attr() to add
the missing

	attr->entry_master = 0;

Fortran testresults on powerpc-ibm-aix5.2.0.0 improve from 1087 unexpected
failures to:

                === gfortran Summary ===

# of expected passes            3934
# of unexpected failures        112
# of unexpected successes       9
# of expected failures          27
# of unsupported tests          8

David