GCC alwayszero function attribute

This is a temporary page for initial thoughts on the project.

• Bug 517370 alwayszero function attribute for gcc

Attempt at identifying attribute implementation

langhooks.h

 /* Pointers to machine-independent attribute tables, for front ends
    using attribs.c.  If one is NULL, it is ignored.  Respectively, a
    table of attributes specific to the language, a table of
    attributes common to two or more languages (to allow easy
    sharing), and a table of attributes for checking formats.  */
 const struct attribute_spec *attribute_table;
 const struct attribute_spec *common_attribute_table;
 const struct attribute_spec *format_attribute_table;