linux-user: Fix MIPS N64 trap and break instruction bug

linux-user: Fix MIPS N64 trap and break instruction bug

For the MIPS N64 ABI when QEMU reads the break/trap instruction so that
it can inspect the break/trap code it reads 8 rather than 4 bytes
which means it finds the code field from the instruction after the
break/trap instruction.  This then causes the break/trap handling
code to fail because it does not understand the code number.

The fix forces QEMU to always read 4 bytes of instruction data rather
than deciding how much to read based on the ABI.

Signed-off-by: Andrew Bennett <andrew.bennett@imgtec.com>
Reviewed-by: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>