x86/unistd: Define X32_NR_syscalls only for 64-bit kernel

X32_NR_syscalls is needed only when building a 64bit kernel.

Move it to proper #ifdef guard.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210517073815.97426-5-masahiroy@kernel.org