vf6: use c99 compatible asm syntax.

using __asm__ instead of asm allows compilation under both gnu89 (old
gcc defaults pre 5.x) and also c99 (gcc 5.x+ defaults)

lib/vf6xx/vector_chipset.c

@@ -27,7 +27,7 @@ static inline void pre_main(void)
 	 * For Vybrid we need to set the stack pointer manually
 	 * since the boot ROM has its own stack
 	 */
-	asm (	\
+	__asm__ (	\
 		"ldr sp,=_stack;" \
 	    );