gdb: Add a find method for RegisterDescriptorIterator

gdb: Add a find method for RegisterDescriptorIterator

Adds a new method 'find' to the gdb.RegisterDescriptorIterator class,
this allows gdb.RegisterDescriptor objects to be looked up directly by
register name rather than having to iterate over all registers.

This will be of use for a later commit.

I've documented the new function in the manual, but I don't think a
NEWS entry is required here, as, since the last release, the whole
register descriptor mechanism is new, and is already mentioned in the
NEWS file.

gdb/ChangeLog:

	* python/py-registers.c: Add 'user-regs.h' include.
	(register_descriptor_iter_find): New function.
	(register_descriptor_iterator_object_methods): New static global
	methods array.
	(register_descriptor_iterator_object_type): Add pointer to methods
	array.

gdb/testsuite/ChangeLog:

	* gdb.python/py-arch-reg-names.exp: Add additional tests.

gdb/doc/ChangeLog:

	* python.texi (Registers In Python): Document new find function.