x86: provide executable fixmap facility

x86: provide executable fixmap facility

This allows us to set aside some address space for executable mapping.
This fixed map range starts from XEN_VIRT_END so that it is within reach
of the .text section.

Shift the percpu stub range and shrink livepatch range accordingly.

Signed-off-by: Wei Liu <liuwe@microsoft.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>