[wp] Slightly improves float constant handling

src/plugins/wp/ProverWhy3.ml

src/plugins/wp/share/why3/frama_c_wp/cfloat.mlw

src/plugins/wp/tests/wp_acsl/float_const.i

src/plugins/wp/tests/wp_acsl/oracle/float_const.res.oracle

...wp/tests/wp_acsl/oracle_qualif/float_compare.0.res.oracle

...nst.0.session/cache/211e52458be3c1ae451dfd1d012b443d.json

...nst.0.session/cache/3744b87afb2193c88857aec75c88378c.json

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...nst.0.session/cache/5d05945c4e22b2861b15d558fb83c6e1.json

...nst.0.session/cache/86231af23e1064da708992cd22fcc6cd.json

...nst.0.session/cache/9c06dc676f02ccbf93885782c64aa8c0.json

...nst.0.session/cache/addd047e7326afb3baae18e98392f6ad.json

...nst.0.session/cache/af03277f3ce7d723e9695176099b9946.json

...ins/wp/tests/wp_acsl/oracle_qualif/float_const.res.oracle