BUILD: debug: mark ha_crash_now() as attribute(noreturn)

BUILD: debug: mark ha_crash_now() as attribute(noreturn)

Building on MIPS64 with clang16 incorrectly reports some uninitialized
value warnings in stats-proxy.c due to some calls to ABORT_NOW() where
the compiler didn't know the code wouldn't return. Let's properly mark
the function as noreturn, and take this opportunity for also marking it
unused to avoid possible warnings depending on the build options (if
ABORT_NOW is not used). No backport needed though it will not harm.

(cherry picked from commit 0a91c6dcaebd41ead51f3654ca96d564ccda32af)
Signed-off-by: Willy Tarreau <w@1wt.eu>