Simplify by using attribute.h macros

Simplify by using attribute.h macros

attribute.h is partly designed for C2X forward compatibility,
since C2X will add some standard attributes.  Using its macros
should help insulate Emacs from C2X teething problems.
* src/conf_post.h: Include attribute.h.
(HAS_ATTRIBUTE, HAS_FEATURE): Rename from __has_attribute and
__has_feature, to avoid polluting the builtin namespace.
All uses changed.
(ATTRIBUTE_COLD, ATTRIBUTE_FORMAT, FALLTHROUGH, ATTRIBUTE_CONST)
(ATTRIBUTE_PURE, ATTRIBUTE_UNUSED, ATTRIBUTE_MAY_ALIAS)
(ATTRIBUTE_MALLOC, ATTRIBUTE_ALLOC_SIZE)
(ATTRIBUTE_RETURNS_NONNULL): Remove, as attribute.h does this now.
(NO_INLINE, EXTERNALLY_VISIBLE, ARG_NONNULL, ATTRIBUTE_UNUSED):
Simplify by defining in terms of attribute.h macros.
* src/systhread.h (ATTRIBUTE_WARN_UNUSED_RESULT): Remove.
All uses replaced by attribute.h’s NODISCARD.