s390: always declare expoline thunks

It would be convenient to use the following pattern in the BPF JIT:

  if (nospec_uses_trampoline())
    emit_call(__s390_indirect_jump_r1);

Unfortunately with CONFIG_EXPOLINE=n the compiler complains about the
missing prototype of __s390_indirect_jump_r1(). One could wrap the
whole "if" statement in an #ifdef, but this clutters the code.

Instead, declare expoline thunk prototypes even when compiling without
expolines. When using the above code structure and compiling without
expolines, references to them are optimized away, and there are no
linker errors.

Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20250519223646.66382-2-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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