Changes

minor gc: more consistent memprof logging

In
  https://github.com/ocaml/ocaml/pull/13580#issuecomment-3092253963
jmid reports that he needed to tweak the GC verbosity setting to avoid
getting spammed by minor-gc messages when debugging an assertion
failure.

The other sub-phases of the GC minor all uses `caml_gc_log` rather
than CAML_GC_MESSAGE, and do not seem to cause similar spamming
issues. Fixing the code to be consistent will avoid inconsistent
verbosity levels in end-user scripts.
 (commit: 69ed94c)