Guile: improved rvalue reference support

Adds a couple of missing bits to the Guile API to make C++11 rvalue
reference values and types usable from Guile scripts.

gdb/ChangeLog:

	* guile/scm-type.c (type_integer_constants): Add binding for
	TYPE_CODE_RVALUE_REF.
	* guile/scm-value.c (gdbscm_value_referenced_value): Handle
	dereferencing of rvalue references.
	* NEWS (Guile API): Note improvements in rvalue reference support.

gdb/doc/ChangeLog:

	* guile.texi (Types In Guile): Add documentation for
	TYPE_CODE_RVALUE_REF.