Revert "x86/alternative: Make custom return thunk unconditional"

This reverts commit 53ebbe1c.

Revert the backport of upstream commit:

  095b8303 ("x86/alternative: Make custom return thunk unconditional")

in order to backport the full version now that

  770ae1b7

 ("x86/returnthunk: Allow different return thunks")

has been backported.

Revert it here so that the build breakage is kept at minimum.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>