arch: fix asm-offsets.c building with -Wmissing-prototypes

arch: fix asm-offsets.c building with -Wmissing-prototypes

When -Wmissing-prototypes is enabled, the some asm-offsets.c files fail
to build, even when this warning is disabled in the Makefile for normal
files:

arch/sparc/kernel/asm-offsets.c:22:5: error: no previous prototype for 'sparc32_foo' [-Werror=missing-prototypes]
arch/sparc/kernel/asm-offsets.c:48:5: error: no previous prototype for 'foo' [-Werror=missing-prototypes]

Address this by making use of the same trick as x86, marking these
functions as 'static __used' to avoid the need for a prototype
by not drop them in dead-code elimination.

Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://lore.kernel.org/lkml/CAK7LNARfEmFk0Du4Hed19eX_G6tUC5wG0zP+L1AyvdpOF4ybXQ@mail.gmail.com/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>