[arch][arm] add arm specific asm header
[arch][arm] disable VFP in irq mode, add arch_in_int_handler

On IRQ entry, disable the floating point unit. If floating point
code is used inside irq mode the undefined handler will panic.
[kernel] add thread_unblock() in case a thread needs to be directly put into ready state

Also tighten the critical section requirement for some of the deep thread
routines for efficiency purposes.