Add --clobber-cache option to initdb, for CCA testing.

Commit 4656e3d66 replaced the "#define CLOBBER_CACHE_ALWAYS"
testing mechanism with a GUC, which has been a great help for
doing cache-clobber testing in more efficient ways; but there
is a gap in the implementation.  The only way to do cache-clobber
testing during an initdb run is to use the old method with #define,
because one can't set the GUC from outside.  Improve this by
adding a switch to initdb for the purpose.

(Perhaps someday we should let initdb pass through arbitrary
"-c NAME=VALUE" switches.  Quoting difficulties dissuaded me
from attempting that right now, though.)

Back-patch to v14 where 4656e3d66 came in.

Discussion: https://postgr.es/m/1582507.1624227029@sss.pgh.pa.us
