bug#51104: comp-tests.elc messes up after reconfiguring for different machine

[Paul Eggert]

When trying out the emacs-28 branch on both 32- and 64-bit platforms, I 
had test failures that I tracked down to this line:

test/src/comp-tests.el:933:       (integer ,most-negative-fixnum 
,most-positive-fixnum))

My problem was that I'd built test/src/comp-tests.elc on a 32-bit 
platform (using "./configure CC='gcc -m32'" on an x86-64 machine) so 
that .elc file had been built assuming 32-bit values for 
most-negative-fixnum and most-positive fixnum. When I then re-ran 
'configure' for a 64-bit platform the .elc files were not automatically 
rebuilt for it (they're supposed to be machine-independent, right?) and 
so the optimizations were incorrect for a 64-bit platform and the tests 
failed.

What's a good way to fix this problem, while still testing everything 
that comp-tests.el wants to test?