x86/returnthunk: Allow different return thunks

x86/returnthunk: Allow different return thunks

Upstream commit: 770ae1b709528a6a173b5c7b183818ee9b45e376

In preparation for call depth tracking on Intel SKL CPUs, make it possible
to patch in a SKL specific return thunk.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220915111147.680469665@infradead.org
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>