dwarf.h: Add DW_AT_noreturn.

dwarf.h: Add DW_AT_noreturn.

DW_AT_noreturn is added by GCC5 for functions that don't return to their
caller. It is set on a DW_TAG_subprogram for the traditional GNU noreturn
attribute, the C11 _Noreturn keyword or C++11 [[noreturn]] attribute. It
is a new DWARF5 extension. This is a constant only addition to dwarf.h.
No other part of elfutils currently depends on this attribute.
Also added to https://fedorahosted.org/elfutils/wiki/DwarfExtensions

Signed-off-by: Mark Wielaard <mjw@redhat.com>