x86/resctrl: Move alloc/mon static keys into helpers

resctrl enables three static keys depending on the features it has enabled.
Another architecture's context switch code may look different, any static keys
that control it should be buried behind helpers.

Move the alloc/mon logic into arch-specific helpers as a preparatory step for
making the rdt_enable_key's status something the arch code decides.

This means other architectures don't have to mirror the static keys.

Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Shaopeng Tan <tan.shaopeng@fujitsu.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Babu Moger <babu.moger@amd.com>
Tested-by: Shaopeng Tan <tan.shaopeng@fujitsu.com>
Tested-by: Peter Newman <peternewman@google.com>
Tested-by: Babu Moger <babu.moger@amd.com>
Tested-by: Carl Worth <carl@os.amperecomputing.com> # arm64
Link: https://lore.kernel.org/r/20240213184438.16675-18-james.morse@arm.com


Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>