x86/asm: Clarify static_cpu_has()'s intended use

Clarify when one should use static_cpu_has() and when one should use
boot_cpu_has().

Requested-by: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: x86@kernel.org
Link: https://lkml.kernel.org/r/20190330112022.28888-2-bp@alien8.de