D62731 implements an incomplete/experimental version of rounding math, but since the flag doesn't have "experimental" in the name, the feedback that it is experimental is in the form of -Wexperimental-float-control.
Since this flag is already being accepted as a gcc compatability arg, put the current implementation being -f[no-]experimental-rounding-math instead, and allow -frounding-math as an ignored arg. Because "experimental" is now in the flag name, remove the warning -- it's now clear that the feature is incomplete/experimental when invoking it.
clang-format: please reformat the code