arches: lib: add main_decl.h for main() declaration

It is silly to have a single header to declare the main()
symbol, however some of the arches provided it while
lib/bootblock.c relied on the arch headers to declare it. Just
move the declaration into its own header file and utilize it.

Change-Id: I743b4c286956ae047c17fe46241b699feca73628
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/13681
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins)

src/include/main_decl.h

@@ -0,0 +1,6 @@
+#ifndef _MAIN_DECL_H_
+#define _MAIN_DECL_H_
+
+void main(void);
+
+#endif