x86: Support indirect thunks from assembly code

x86: Support indirect thunks from assembly code

Introduce INDIRECT_CALL and INDIRECT_JMP which either degrade to a normal
indirect branch, or dispatch to the __x86_indirect_thunk_* symbols.

Update all the manual indirect branches in to use the new thunks.  The
indirect branches in the early boot and kexec path are left intact as we can't
use the compiled-in thunks at those points.

This is part of XSA-254.

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