modpost: define TO_NATIVE() using bswap_* functions
The current TO_NATIVE() has some limitations:
1) You cannot cast the argument.
2) You cannot pass a variable marked as 'const'.
3) Passing an array is a bug, but it is not detected.
Impelement TO_NATIVE() using bswap_*() functions. These are GNU
extensions. If we face portability issues, we can port the code from
include/uapi/linux/swab.h.
With this change, get_rel_type_and_sym() can be simplified by casting
the arguments directly.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>