- removed the __attribute__((naked)) from ISRs
- removed ISR_ENTER() and ISR_EXIT() macros
Rationale: Cortex-Mx MCUs save registers R0-R4 automatically
on calling ISRs. The naked attribute tells the compiler not
to save any other registers. This is fine, as long as the
code in the ISR is not nested. If nested, it will use also
R4 and R5, which will then lead to currupted registers on
exit of the ISR. Removing the naked will fix this.
In many places we needlessly use `sched_active_thread->pid` whilst we
already have `sched_active_pid` with the same value, and one less
indirection.
`thread_getpid()` is made `static inline` so that there is no penalty in
using this function over accessing `sched_active_pid` directly.
