[XCOFF] write the aux header when the visibility is specified in XCOFF32.

The n_type field in the symbol table entry has two interpretations in XCOFF32, and a single interpretation in XCOFF64.
The new interpretation is used in XCOFF32 if the value of the o_vstamp field in the auxiliary header is 2.
In XCOFF64 and the new XCOFF32 interpretation, the n_type field is used for the symbol type and visibility.
The patch writes the aux header with an o_vstamp field value of 2 when the visibility is specified in XCOFF32 to make the new XCOFF32 interpretation used.

Reviewed By: DiggerLin, jhenderson

Differential Revision: https://reviews.llvm.org/D128148