genksyms: decouple ATTRIBUTE_PHRASE from type-qualifier

The __attribute__ keyword can appear in more contexts than 'const' or
'volatile'.

To avoid grammatical conflicts with future changes, ATTRIBUTE_PHRASE
should not be reduced into type_qualifier.

No functional changes are intended.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Nicolas Schier <n.schier@avm.de>