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/arch/arm/include/arch/stages.h

@@ -16,7 +16,7 @@
 #ifndef __ARCH_STAGES_H
 #define __ARCH_STAGES_H
 
-extern void main(void);
+#include <main_decl.h>
 
 void stage_entry(void);
 

src/arch/arm64/include/arch/stages.h

@@ -17,8 +17,7 @@
 #define __ARCH_STAGES_H
 
 #include <stdint.h>
-
-extern void main(void);
+#include <main_decl.h>
 
 void stage_entry(void);
 

src/arch/mips/include/arch/stages.h

@@ -16,7 +16,7 @@
 #ifndef __MIPS_ARCH_STAGES_H
 #define __MIPS_ARCH_STAGES_H
 
-extern void main(void);
+#include <main_decl.h>
 
 void stage_entry(void);
 

src/arch/riscv/include/arch/stages.h

@@ -16,7 +16,7 @@
 #ifndef __ARCH_STAGES_H
 #define __ARCH_STAGES_H
 
-extern void main(void);
+#include <main_decl.h>
 
 void stage_entry(void) __attribute__((section(".text.stage_entry")));