add -fvisibility=hidden to CC flags, mark XXX_main functions

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

shell/ash.c

@@ -12739,7 +12739,7 @@ extern int etext();
  * exception occurs.  When an exception occurs the variable "state"
  * is used to figure out how far we had gotten.
  */
-int ash_main(int argc, char **argv);
+int ash_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int ash_main(int argc, char **argv)
 {
 	char *shinit;

shell/bbsh.c

@@ -198,7 +198,7 @@ static void handle(char *command)
 	}
 }
 
-int bbsh_main(int argc, char **argv);
+int bbsh_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int bbsh_main(int argc, char **argv)
 {
 	char *command=NULL;

shell/hush.c

@@ -3675,7 +3675,7 @@ static void setup_job_control(void)
 }
 #endif
 
-int hush_main(int argc, char **argv);
+int hush_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int hush_main(int argc, char **argv)
 {
 	static const char version_str[] ALIGN1 = "HUSH_VERSION="HUSH_VER_STR;

shell/lash.c

@@ -1491,7 +1491,7 @@ static inline void setup_job_control(void)
 }
 #endif
 
-int lash_main(int argc_l, char **argv_l);
+int lash_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int lash_main(int argc_l, char **argv_l)
 {
 	unsigned opt;

shell/msh.c

@@ -5164,7 +5164,7 @@ static void freehere(int area)
  * shell
  */
 
-int msh_main(int argc, char **argv);
+int msh_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int msh_main(int argc, char **argv)
 {
 	int f;