rm48x50: utilize CPS instruction and remove some useless code
rm48x50: optimize context_switch_interrupt_do

Substitude STMFD, MOV, ADD with STMFD, SUB. It reduce one instruction.
Tested on board and it works like a charm.
rm48x50: add reg_test from FreeRTOS

The code is GPLv2 so I think we could use it for free(both free beer and
free speech).
rm48x50: fix bug in reg_test from FreeRTOS

It does not satisfy AAPCS.
rm48x50: add vRegTestTask2
rm48x50: temperately disable the VFP register test
rm48x50: optimize context_switch_interrupt_to by reuse registers

When saving thread registers in context_switch_interrupt_to, we don't
change them, just move them. So there is no need to always r0-r3 from
stack to the real r0-r3. So just use the intermediate registers and
eliminate 2 MOV.