x86/spec-ctrl: Software BHB-clearing sequences

x86/spec-ctrl: Software BHB-clearing sequences

Implement clear_bhb_{tsx,loops}() as per the BHI guidance.  The loops variant
is set up as the "short" sequence.

Introduce SCF_entry_bhb and extend SPEC_CTRL_ENTRY_* with a conditional call
to selected clearing routine.

Note that due to a limitation in the ALTERNATIVE capability, the TEST/JZ can't
be included alongside a CALL in a single alternative block.  This is going to
require further work to untangle.

The BHB sequences (if used) must be after the restoration of Xen's
MSR_SPEC_CTRL value, which must be accounted for when judging whether it is
safe to skip the safety LFENCEs.

This is part of XSA-456 / CVE-2024-2201.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>