mips: Ensure PC update with MTC0 single-stepping

Correct the way PC is updated when single-stepping instructions, by
keeping the old PC only for the BS_EXCP (exception condition) state.

Some MTC0 (and possibly other) instructions switch to the BS_STOP state
to terminate the current translation block, so that the state transition
of the simulated CPU resulting from the CP0 operation takes effect with
the following instruction.  This happens with `mtc0 <reg>,c0_config' for
example, typically used to set KSEG0 cacheability.

While single-stepping this has a side-effect of not advancing the PC
past the instruction just executed; subsequent single-step traps will
stop at the same instruction repeatedly.  Example:

(gdb) stepi
0x80004d24 in _start ()
5: x/i $pc
=> 0x80004d24 <_start+364>:     mfc0    t1,c0_config
(gdb)
0x80004d28 in _start ()
5: x/i $pc
=> 0x80004d28 <_start+368>:     li      at,-8
(gdb)
0x80004d2c in _start ()
5: x/i $pc
=> 0x80004d2c <_start+372>:     and     t1,t1,at
(gdb)
0x80004d30 in _start ()
5: x/i $pc
=> 0x80004d30 <_start+376>:     ori     t1,t1,0x3
(gdb)
0x80004d34 in _start ()
5: x/i $pc
=> 0x80004d34 <_start+380>:     mtc0    t1,c0_config
(gdb)
0x80004d34 in _start ()
5: x/i $pc
=> 0x80004d34 <_start+380>:     mtc0    t1,c0_config
(gdb)
0x80004d34 in _start ()
5: x/i $pc
=> 0x80004d34 <_start+380>:     mtc0    t1,c0_config
(gdb)
0x80004d34 in _start ()
5: x/i $pc
=> 0x80004d34 <_start+380>:     mtc0    t1,c0_config
(gdb)

-- oops!

Signed-off-by: Maciej W. Rozycki <macro@codesourcery.com>
Reviewed-by: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
