Add a new method 'disassemble' to gdb.Architecture class.

	* python/py-arch.c (archpy_disassmble): Implementation of the
	new method gdb.Architecture.disassemble.
	(arch_object_methods): Add entry for the new method.

	* doc/gdb.texinfo (Architectures In Python): Add description
	about the new method gdb.Architecture.disassemble.

	* testsuite/gdb.python/py-arch.c: New test case
	* testsuite/gdb.python/py-arch.exp: New tests to test
	gdb.Architecture.disassemble
	* testsuite/gdb.python/Makefile.in: Add py-arch to the list of
	EXECUTABLES.