Avoid compiler warnings from MSVC in generated code.

runtime/mercury_float.h:
	Avoid warnings from MSVC about the types of the formal and actual parameters
	of calls to MR_float_from_dword differing.  The formal parameters have type
	MR_Word, but the Mercury compiler doesn't emit casts for the arguments.
	(For GCC and clang, MR_float_from_dword is defined as a macro and the
	necessary casts occur in its body.)  The fix is to rename the function
	version of MR_float_from_dword and make MR_float_from_dword a forwarding macro
	that inserts the necessary casts.