[libc] Use __builtin_va_list et al in stdarg.h

The home-grown versions don't work properly for static variadic
functions, when gcc can choose to use a non-standard calling convention.

src/arch/i386/include/stdarg.h

@@ -1,22 +0,0 @@
-#ifndef _STDARG_H
-#define _STDARG_H
-
-typedef void * va_list;
-
-#define va_start( ap, last ) do {	\
-		ap = ( &last + 1 );	\
-	} while ( 0 )
-
-#define va_arg( ap, type ) ({		\
-		type *_this = ap;	\
-		ap = ( _this + 1 );	\
-		*(_this);		\
-	})
-
-#define va_end( ap ) do { } while ( 0 )
-
-#define va_copy( dest, src ) do {	\
-		dest = src;		\
-	} while ( 0 )
-
-#endif /* _STDARG_H */