Disable diagnostic 1558 (--float_operations_allowed diagnostic) in standard header files

The --float_operations_allowed option is used to identify floating point operations and issue an error if the user doesn't want them in the program. A program that includes math.h will generate diagnostics if either none or 32 is used as an argument. The emitting of the diagnostics is probably OK for none since users really shouldn't include math.h if no floating point is to be done, but it is a problem for 32-bit operations. If a user wants to only use 32-bit floating point, it is reasonable for them to include math.h. We should suppress the diagnostics from the standard header files.