pgsql: Clean up unportable attempt to use #if inside a macro call, also

Log Message:
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Clean up unportable attempt to use #if inside a macro call, also
faulty code in third arm of #if. Per buildfarm reports.

Modified Files:
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pgsql/src/backend/postmaster:
pgarch.c (r1.34 -> r1.35)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgarch.c?r1=1.34&r2=1.35)