fs: Build sys_stat64() and friends if __ARCH_WANT_COMPAT_STAT64

On AArch64 Linux, we want the sys_stat64() and related functions for
compat support but do not need the generic struct stat64, enabled
automatically if __ARCH_WANT_STAT64.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>