Make cast between words and floats real primops (#24331)

compiler/GHC/Builtin/primops.txt.pp

compiler/GHC/StgToCmm/Prim.hs

compiler/GHC/StgToJS/Prim.hs

libraries/ghc-internal/cbits/CastFloatWord.cmm

libraries/ghc-internal/ghc-internal.cabal

libraries/ghc-internal/src/GHC/Float.hs

rts/PrimOps.cmm

rts/RtsSymbols.c

rts/include/stg/MiscClosures.h

rts/js/arith.js

testsuite/tests/interface-stability/base-exports.stdout

...ce-stability/base-exports.stdout-javascript-unknown-ghcjs

...ite/tests/interface-stability/base-exports.stdout-mingw32

...suite/tests/interface-stability/base-exports.stdout-ws-32

testsuite/tests/numeric/should_compile/T24331.hs

testsuite/tests/numeric/should_compile/T24331.stderr

testsuite/tests/numeric/should_compile/all.T