x86/alternative: Add indirect call patching

In order to prepare replacing of paravirt patching with alternative
patching, add the capability to replace an indirect call with a direct
one.

This is done via a new flag ALT_FLAG_CALL as the target of the CALL
instruction needs to be evaluated using the value of the location
addressed by the indirect call.

For convenience, add a macro for a default CALL instruction. In case it
is being used without the new flag being set, it will result in a BUG()
when being executed. As in most cases, the feature used will be
X86_FEATURE_ALWAYS so add another macro ALT_CALL_ALWAYS usable for the
flags parameter of the ALTERNATIVE macros.

For a complete replacement, handle the special cases of calling a nop
function and an indirect call of NULL the same way as paravirt does.

  [ bp: Massage commit message, fixup the debug output and clarify flow
    more. ]

Co-developed-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20231210062138.2417-4-jgross@suse.com