x86, generic: mark early_printk as asmlinkage

It's not explicitly marked as asmlinkage, but invoked from x86_32
startup code with parameters on stack.

No other architectures define early_printk and none of them are affected
by this change, since defines asmlinkage as empty token.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/printk.c

@@ -38,7 +38,7 @@
 /*
  * Architectures can override it:
  */
-void __attribute__((weak)) early_printk(const char *fmt, ...)
+void asmlinkage __attribute__((weak)) early_printk(const char *fmt, ...)
 {
 }