testsuite: Fix gdb.base/bang.exp for remote stubs without exit

Some remote stubs do not have a proper exit() function implementation.
gdb.base/bang.exp was failing on those targets due to timeout. With this
patch bang.exp uses already defined library procedures to handle this
situation gracefully without breaking native targets.

Tested with x86_64 (unix, native-gdbserver) and with arc-*-elf32.

gdb/testsuite/ChangeLog:

2013-10-08  Anton Kolesov <Anton.Kolesov@synopsys.com>

	* gdb.base/bang.exp: Use gdb_continue_to_end to properly support
	remote stubs where exit() behaviour is unreliable.

Signed-off-by: Anton Kolesov <akolesov@synopsys.com>