arm/x86/common: Add HAS_[ALTERNATIVE|EX_TABLE]

arm/x86/common: Add HAS_[ALTERNATIVE|EX_TABLE]

x86 implements all of them by default - and we just
add two extra HAS_ variables to be declared in autoconf.h.

ARM 64 only has alternative while ARM 32 has none of them.

And while at it change the livepatch common code that
would benefit from this.

Reviewed-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com> [relevant parts]
Suggested-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>