tools/nolibc: compiler: add macro __nolibc_fallthrough

tools/nolibc: compiler: add macro __nolibc_fallthrough

Recent version of GCC and clang gained -Wimplicit-fallthrough,
warning about implicit fall-through between switch labels.
As nolibc does not control the compilation flags, this can trigger
warnings for when built by the user.
Make use of the "fallthrough" attribute to explicitly annotate the
expected fall-throughs and silence the warning.

Link: https://lore.kernel.org/r/20240930-nolibc-fallthrough-v2-1-2e8d10fe3430@weissschuh.net
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>