MIPS: Fall back to generic implementation of cmpxchg64 on 32-bit platforms

This is in preparation of adding HAVE_VIRT_CPU_ACCOUNTING_GEN support in
the next patch.

Without having cmpxchg64 to use the generic implementation, kernel linking
will complain:

kernel/built-in.o: In function `cputime_adjust':
cputime.c:(.text+0x33748): undefined reference to `__cmpxchg_called_with_bad_pointer'
cputime.c:(.text+0x33810): undefined reference to `__cmpxchg_called_with_bad_pointer'

Signed-off-by: Deng-Cheng Zhu <dengcheng.zhu@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: macro@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9474/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>