fallback_casx build error on s390x

[Paul Cercueil]

In file included from deps/lightning/lib/jit_s390.c:1570,
                from deps/lightning/lib/lightning.c:3700:
deps/lightning/lib/jit_fallback.c: In function '_fallback_casx':
deps/lightning/lib/jit_fallback.c:157:10: error: void value not ignored 
as it ought to be
 157 | jump = bnei(_jit->pc.w, r0, 1);
     | ^
deps/lightning/lib/lightning.c: In function '_redundant_store':
deps/lightning/lib/lightning.c:3098:1: warning: control reaches end of 
non-void function [-Wreturn-type]
3098 | }
     | ^
make: *** [Makefile:394: deps/lightning/lib/lightning.o] Error 1


It looks to me like jit_fallback.c expects "bnei" to return a 
jit_word_t. Shouldn't it use bnei_p() instead? But then some archs 
(e.g. MIPS) don't provide bnei_p().

The warning is also a bit concerning.

Cheers,
-Paul