x86/lib: Add WBNOINVD helper functions

In line with WBINVD usage, add WBNOINVD helper functions.  Explicitly fall
back to WBINVD (via alternative()) if WBNOINVD isn't supported even though
the instruction itself is backwards compatible (WBNOINVD is WBINVD with an
ignored REP prefix), so that disabling X86_FEATURE_WBNOINVD behaves as one
would expect, e.g. in case there's a hardware issue that affects WBNOINVD.

Opportunistically, add comments explaining the architectural behavior of
WBINVD and WBNOINVD, and provide hints and pointers to uarch-specific
behavior.

Note, alternative() ensures compatibility with early boot code as needed.

  [ bp: Massage, fix typos, make export _GPL. ]

Signed-off-by: Kevin Loughlin <kevinloughlin@google.com>
Co-developed-by: Sean Christopherson <seanjc@google.com>
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>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/20250522233733.3176144-4-seanjc@google.com