x86/altcall: Introduce new simpler scheme

x86/altcall: Introduce new simpler scheme

Encoding altcalls as regular alternatives leads to an unreasonable amount of
complexity in _apply_alternatives().

Introduce apply_alt_calls(), and an .alt_call_sites section which simply
tracks the source address (relative, to save on space).  That's literally all
that is needed in order to devirtualise the function pointers.

apply_alt_calls() is mostly as per _apply_alternatives(), except the size is
known to be 6 bytes.  Drop the logic for JMP *RIPREL, as there's no support
for tailcall optimisations, nor a feasbile plan on how to introduce support.
Pad with a redundant prefix to avoid needing a separate NOP on the end.

Wire it up in nmi_apply_alternatives(), although the section is empty at this
juncture so nothing happens in practice.

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