x86/lib: Add WBINVD and WBNOINVD helpers to target multiple CPUs

Extract KVM's open-coded calls to do writeback caches on multiple CPUs to
common library helpers for both WBINVD and WBNOINVD (KVM will use both).
Put the onus on the caller to check for a non-empty mask to simplify the
SMP=n implementation, e.g. so that it doesn't need to check that the one
and only CPU in the system is present in the mask.

  [sean: move to lib, add SMP=n helpers, clarify usage]

Signed-off-by: Zheyun Shen <szy0127@sjtu.edu.cn>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Acked-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20250128015345.7929-2-szy0127@sjtu.edu.cn
Link: https://lore.kernel.org/20250522233733.3176144-5-seanjc@google.com