x86: infrastructure to allow converting certain indirect calls to direct ones

x86: infrastructure to allow converting certain indirect calls to direct ones

In a number of cases the targets of indirect calls get determined once
at boot time. In such cases we can replace those calls with direct ones
via our alternative instruction patching mechanism.

Some of the targets (in particular the hvm_funcs ones) get established
only in pre-SMP initcalls, making necessary a second passs through the
alternative patching code. Therefore some adjustments beyond the
recognition of the new special pattern are necessary there.

Note that patching such sites more than once is not supported (and the
supplied macros also don't provide any means to do so).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>