This commit also refactors how geometric builtins are defined and
declared, by sharing more helpers. It also removes an unnecessary
gentype-like helper in favour of the more complete math/gentype.inc.
There are no changes to the IR for any of these four builtins.
The 'normalize' builtin will follow in a subsequent commit because it
would involve the addition of missing halfn-type overloads for
completeness.
655151a7e0