CODE_OWNERS: update information for builtins

Mark myself as the code owner for the builtins library as per the
discussion at
https://discourse.llvm.org/t/code-owner-for-compiler-rt-builtins-library.

Differential Revision: https://reviews.llvm.org/D124263
Reviewed By: aaron.ballman, lattner

GitOrigin-RevId: 07513605b2231b4046d7827ac572ee1205b867b1
1 file changed