target-mips: Correct MIPS16/microMIPS branch size calculation

target-mips: Correct MIPS16/microMIPS branch size calculation

Correct MIPS16/microMIPS branch size calculation in PC adjustment
needed:

- to set the value of CP0.ErrorEPC at the entry to the reset exception,

- for the purpose of branch reexecution in the context of device I/O.

Follow the approach taken in `exception_resume_pc' for ordinary, Debug
and NMI exceptions.

MIPS16 and microMIPS branches can be 2 or 4 bytes in size and that has
to be reflected in calculation.  Original MIPS ISA branches, which is
where this code originates from, are always 4 bytes long, just as all
original MIPS ISA instructions.

Signed-off-by: Nathan Froyd <froydnj@codesourcery.com>
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>