MFC r330110: Add kernel retpoline option for amd64

Retpoline is a compiler-based mitigation for CVE-2017-5715, also known
as Spectre V2, that protects against speculative execution branch target
injection attacks.

In this commit it is disabled by default, but will be changed in a
followup commit.

MFC r330962: Remove KERNEL_RETPOLINE from BROKEN_OPTIONS on i386

Clang will compile both amd64 and i386 with retpoline.

Sponsored by: The FreeBSD Foundation