x86/mm: Add global ASID allocation helper functions

Add functions to manage global ASID space. Multithreaded processes that are
simultaneously active on 4 or more CPUs can get a global ASID, resulting in the
same PCID being used for that process on every CPU.

This in turn will allow the kernel to use hardware-assisted TLB flushing
through AMD INVLPGB or Intel RAR for these processes.

  [ bp:
   - Extend use_global_asid() comment
   - s/X86_BROADCAST_TLB_FLUSH/BROADCAST_TLB_FLUSH/g
   - other touchups ]

Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250226030129.530345-8-riel@surriel.com