Fix "control reaches end of non-void function" errors in testsuite

When running the testsuite with clang, a number of testcases fail to
build with the following errors:
  warning: control reaches end of non-void function [-Wreturn-type]
  warning: control may reach end of non-void function [-Wreturn-type]

This prevents a number of testcases from executing.  This commit fixes.

gdb/testsuite/ChangeLog:

	* gdb.base/info-os.c (main): Add return statement.
	* gdb.base/info_minsym.c (minsym_fun): Likewise.
	* gdb.base/large-frame-2.c (func): Likewise.
	* gdb.base/pr10179-a.c (foo1, bar1): Likewise.
	* gdb.base/pr10179-b.c (foo2): Likewise.
	* gdb.base/valgrind-disp-step.c (foo): Likewise.
	* gdb.base/watch-cond.c (func): Likewise.
	* gdb.multi/goodbye.c (verylongfun): Likewise.
	* gdb.multi/hello.c (commonfun): Likewise.
	* gdb.python/py-finish-breakpoint.c (call_longjmp): Likewise.
	* gdb.threads/fork-plus-threads.c (thread_func): Likewise.
	* gdb.threads/forking-threads-plus-breakpoint.c (thread_forks):
	Likewise.
	* gdb.threads/hand-call-new-thread.c (foo): Likewise.
	* gdb.threads/interrupt-while-step-over.c (child_function):
	Likewise.
	* gdb.trace/actions-changed.c (end): Likewise.

gdb/testsuite/ChangeLog

@@ -1,3 +1,23 @@
+2020-05-15  Gary Benson <gbenson@redhat.com>
+
+	* gdb.base/info-os.c (main): Add return statement.
+	* gdb.base/info_minsym.c (minsym_fun): Likewise.
+	* gdb.base/large-frame-2.c (func): Likewise.
+	* gdb.base/pr10179-a.c (foo1, bar1): Likewise.
+	* gdb.base/pr10179-b.c (foo2): Likewise.
+	* gdb.base/valgrind-disp-step.c (foo): Likewise.
+	* gdb.base/watch-cond.c (func): Likewise.
+	* gdb.multi/goodbye.c (verylongfun): Likewise.
+	* gdb.multi/hello.c (commonfun): Likewise.
+	* gdb.python/py-finish-breakpoint.c (call_longjmp): Likewise.
+	* gdb.threads/fork-plus-threads.c (thread_func): Likewise.
+	* gdb.threads/forking-threads-plus-breakpoint.c (thread_forks):
+	Likewise.
+	* gdb.threads/hand-call-new-thread.c (foo): Likewise.
+	* gdb.threads/interrupt-while-step-over.c (child_function):
+	Likewise.
+	* gdb.trace/actions-changed.c (end): Likewise.
+
 2020-05-15  Gary Benson <gbenson@redhat.com>
 
 	* gdb.opencl/callfuncs.exp: Report when test skipped.

gdb/testsuite/gdb.base/info-os.c

@@ -48,6 +48,8 @@ thread_proc (void *args)
 {
   pthread_mutex_lock (&mutex);
   pthread_mutex_unlock (&mutex);
+
+  return NULL;
 }
 
 int

gdb/testsuite/gdb.base/info_minsym.c

@@ -20,6 +20,7 @@ static int minsym_var;
 static int minsym_fun (void)
 {
    minsym_var++;
+   return 0;
 }
 
 int

gdb/testsuite/gdb.base/large-frame-2.c

@@ -22,4 +22,5 @@ func (void)
 {
   int a[4096];
   blah (a);
+  return 0;
 }

gdb/testsuite/gdb.base/pr10179-a.c

@@ -5,11 +5,13 @@ extern int foo2();
 int
 foo1()
 {
+  return 0;
 }
 
 int
 bar1()
 {
+  return 0;
 }
 
 int

gdb/testsuite/gdb.base/pr10179-b.c

@@ -3,4 +3,5 @@
 int
 foo2()
 {
+  return 0;
 }

gdb/testsuite/gdb.base/valgrind-disp-step.c

@@ -18,6 +18,7 @@
 static int
 foo (void)
 {
+  return 0;
 }
 
 int

gdb/testsuite/gdb.base/watch-cond.c

@@ -23,6 +23,7 @@ int func(int *foo)
   (*foo)++;
   global++;
   global2++;
+  return 0;
 }
 
 void func2(int *foo)

gdb/testsuite/gdb.multi/goodbye.c

@@ -37,6 +37,7 @@ int verylongfun()
   glob *= 8;
   glob += 9;
   glob *= 9;
+  return 0;
 }
 
 void

gdb/testsuite/gdb.multi/hello.c

@@ -29,7 +29,7 @@ bar()
     exit(1);
 }
 
-int commonfun() { bar(); } /* from hello */
+int commonfun() { bar(); return 0; } /* from hello */
 
 int
 hello(int x)

gdb/testsuite/gdb.python/py-finish-breakpoint.c

@@ -56,6 +56,7 @@ int
 call_longjmp (jmp_buf *buf)
 {
   call_longjmp_1 (buf);
+  return 0;
 }
 
 void

gdb/testsuite/gdb.threads/fork-plus-threads.c

@@ -33,6 +33,7 @@ static void *
 thread_func (void *arg)
 {
   /* Empty.  */
+  return NULL;
 }
 
 static void

gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.c

@@ -90,6 +90,8 @@ thread_forks (void *arg)
 	  exit (1);
 	}
     }
+
+  return NULL;
 }
 
 /* Set this to tell the thread_breakpoint thread to exit.  */

gdb/testsuite/gdb.threads/hand-call-new-thread.c

@@ -24,6 +24,7 @@ static int
 foo (void)
 {
   usleep (1);
+  return 0;
 }
 
 static void *

gdb/testsuite/gdb.threads/interrupt-while-step-over.c

@@ -44,6 +44,8 @@ child_function (void *arg)
   pthread_barrier_wait (&threads_started_barrier);
 
   infinite_loop ();
+
+  return NULL;
 }
 
 void

gdb/testsuite/gdb.trace/actions-changed.c

@@ -18,6 +18,7 @@
 int
 end (int i)
 {
+  return 0;
 }
 
 int