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

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.