Support float number literal with leading zeroes
builtin.h: Only #define __stdcall & co for Win32

The old FMOD 3 headers have #ifdef __stdcall checks to detect Win32
as opposed to Linux. So it's best to follow the gcc behaviour exactly.
builtin.h: Move non-compiler-defines into limits.h
Save number literals as strings internally again

This way we can preserve the exact spelling, e.g. leading or trailing
zeroes and hex literal case. This also improves the handling of float
literals: They're preserved as-is, without any conversions that may lose
precision etc.

It's fairly easy to do this now, since fbfrog rarely needs to know the
value of a number literal (no more CONST folding being done). The main
place remaining is the CPP's expression evaluation.
Stop treating __cdecl/__stdcall as keywords

They're just pre-#defines under MinGW, not "universal" keywords though.
Automatically strip __attribute__ #defines