A note says "The nomips16 attribute should not really be needed,
it works around a toolchain issue in 2016.05-03."
In fact, in 2018.09-03, the attribute leads to this error:
```cpu/mips32r2_common/thread_arch.c:191:1: error: ‘_mips_handle_exception’ redeclared with conflicting ‘nomips16’ attributes```
This this commit removes the attribute.
- removed ISR_STACKSIZE define where unused (set to 0)
- removed thread_arch_isr_stack_usage(), thread_arch_isr_stack_start(),
and/or thread_arch_isr_stack_pointer() where not implemented
cpu/mips32r2_common adds base architecture support for mips32r2 cores it can be
built in it own right as a 'CPU', but is dependant on a bootloader (like
u-boot) to have bootstrapped the system, this has been tested on a 'malta'
FPGA system (BOARD=mips-malta) with various mips32r2 compliant cores
(interAptiv, P5600, etc).
