Fix compilation errors with clang in gdb.base/advance.c

Fix compilation errors with clang in gdb.base/advance.c

Clang fails to compile the above file, with the following errors:
  warning: control reaches end of non-void function [-Wreturn-type]
  warning: too many arguments in call to 'func'

This prevents the following testcases from executing:
  gdb.base/advance.exp
  gdb.base/until-nodebug.exp

gdb/testsuite/ChangeLog:

	* gdb.base/advance.c (func): New argument, to match call site.
	(func2, func3): Add return statements.