x86/ucode: Use altcall, and __initconst_cf_clobber

x86/ucode: Use altcall, and __initconst_cf_clobber

Microcode loading is not a fastpath, but there are control flow integrity
hardening benefits from using altcall, because it allows us to clobber the
endbr64 instructions on all function pointer targets.

Convert the existing microcode_ops pointer into an __ro_after_init structure,
and move {amd,intel}_ucode_ops into __initconst_cf_clobber.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>