Disable WITH_RETPOLINE on stable/12.

It is known that clang 7 generates broken ifunc calls when retpoline is
enabled. Since libc uses ifuncs, mark WITH_RETPOLINE as always broken
and disabled. To be removed after clang 8 MFC.

This is a direct commit to the branch.

Reported by: many
Tested by: Nikola Mihaylov <c.source.code@gmail.com>
Reviewed by: emaste
Sponsored by: The FreeBSD Foundation