MIPS: Loongson 2F: Fix of problems introduced by -mfix-loongson2f-jump

MIPS: Loongson 2F: Fix of problems introduced by -mfix-loongson2f-jump

The -mfix-loongson2f-jump option provided by latest CVS binutils have fixed
the out-of-order issue of Loongson-2F described in chapter 15 of the
Loongson2F User Manual [1, 2], but introduced some problems.

The option changes all of the jump target to "addr & 0xcfffffff" through the
at($1) register, but for the reboot address of Loongson 2F 0xbfc00000 this is
wrong.  Avoids the problem via telling the assembler to not use the $at
register.

[1] Loongson2F User Manual (Chinese Version)
    http://www.loongson.cn/uploadfile/file/200808211
[2] English Version of Chapter 15:
    http://groups.google.com.hk/group/loongson-dev/msg/e0d2e220958f10a6?dmode=source

Reported-and-tested-by: Liu Shiwei <liushiwei@gmail.com>
Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips <linux-mips@linux-mips.org>
Patchwork: http://patchwork.linux-mips.org/patch/1109/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>