AIX: Fix missing libpq symbols by respecting SHLIB_EXPORTS.

We make each AIX shared library export all globals found in .o files
that originate in the library.  That doesn't include symbols acquired by
-lpgcommon_shlib.  That is good on average, but it became a problem for
libpq when commit e6afa8918c461c1dd80c5063a950518fa4e950cd moved five
official libpq API symbols into src/common.  Fix this by implementing
the SHLIB_EXPORTS mechanism for AIX, so affected libraries export the
same symbols that they export on Linux.  This reintroduces symbols
pg_encoding_to_char, pg_utf_mblen, pg_char_to_encoding,
pg_valid_server_encoding, and pg_valid_server_encoding_id.  Back-patch
to v13, where the aforementioned commit first appeared.  While a minor
release is usually the wrong time to add or remove symbol exports in
libpq or libecpg, we should expect users to want each documented symbol.

Tony Reix

Discussion: https://postgr.es/m/PR3PR02MB6396742E2FC3E77D37A920BC86C79@PR3PR02MB6396.eurprd02.prod.outlook.com
