target-mips: Use EXCP_SC rather than a magic number

  From the discussion on the ML [1], the exception limit defined by
magic number 0x100 is actually EXCP_SC defined in cpu.h. Replace the
magic number with EXCP_SC. Remove "#if 1 .. #endif" as well.

[1] http://lists.gnu.org/archive/html/qemu-devel/2012-11/msg03080.html

Signed-off-by: Chen Wei-Ren <chenwj@iis.sinica.edu.tw>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
