unicore32: Fix __ARCH_WANT_STAT64 definition

The __ARCH_WANT_STAT64 macro must be defined before including
asm-generic/unistd.h. I got this right for everything except
unicore32.

Fixes: bf4b6a7d ("y2038: Remove stat64 family from default syscall set")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>