Support gcc -fkeep-inline-functions

Support gcc -fkeep-inline-functions

For some systems, we need to avoid unsatisfied-external-reference
errors in static inlines.  See
27d2693187d1bcf2563ee7142ba37d4788c8d52b for example.  In order to
test that on other systems, the gcc option -fkeep-inline-functions can
be used.  But it actually is a bit stricter than what we currently
have in place, so fix up a few more places along the lines of the
above commit.  (This undoes part of commit
2cd2569c72b8920048e35c31c9be30a6170e1410.)

(Note, this does not add that gcc option anywhere to the build system,
it just makes it possible to use it successfully manually.)

Discussion: https://www.postgresql.org/message-id/flat/E1oBgIW-002ehP-VJ%40gemulon.postgresql.org