x86/bhi: Add BHI stubs

Add an array of code thunks, to be called from the FineIBT preamble,
clobbering the first 'n' argument registers for speculative execution.

Notably the 0th entry will clobber no argument registers and will never
be used, it exists so the array can be naturally indexed, while the 7th
entry will clobber all the 6 argument registers and also RSP in order to
mess up stack based arguments.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Link: https://lore.kernel.org/r/20250224124200.717378681@infradead.org