Instead of using `weak` function definitions, this PR handles
default implementations using `PROVIDES_x` defines, allowing
for cpus/pm realted modules to use their own implementations.
- 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).
