make -d:nimFpRoundtrips work consistently in vm vs rt, fix #18400, etc (#18531)

* compiler/vmhooks: add getVar to allow vmops with var params
* addFloat vmops with var param
* cgen now renders float32 literals in c backend using roundtrip float to string

compiler/semfold.nim

@@ -290,7 +290,6 @@ proc evalOp(m: TMagic, n, a, b, c: PNode; idgen: IdGenerator; g: ModuleGraph): P
   of mBoolToStr:
     if getOrdValue(a) == 0: result = newStrNodeT("false", n, g)
     else: result = newStrNodeT("true", n, g)
-  of mFloatToStr: result = newStrNodeT($getFloat(a), n, g)
   of mCStrToStr, mCharToStr:
     if a.kind == nkBracket:
       var s = ""