Introduce Cutil.class_of_attribute to return the class of the given attribute: one among
Attr_type  attribute related to types  (e.g. "aligned")
Attr_struct attribute related to struct/union/enum types (e.g. "packed")
Attr_function attribute related to function types (e.g. "noreturn")
Attr_name  attribute related to variable and function declarations (e.g. "section")
Attr_unknown  attribute was not declared

Cutil.declare_attribute is used to associate a class to a custom attribute.
Standard attributes (const, volatile, _Alignas, etc) are Attr_type.

cfronted/C2C.ml: declare the few attributes that CompCert honors currently.
cparser/GCC.ml: a bigger list of attributes taken from GCC, for reference only.