The VRSAVE feature of the Altivec unit can be used to reduce the amount of
Altivec registers which need to be saved/restored during interrupt processing
and context switches.
In order to use the VRSAVE optimization a corresponding multilib (-mvrsave) is
required, see GCC configuration. The -mvrsave option must be added to the
ABI_FLAGS of the BSP.
Currently only the -mcpu=e6500 based QorIQ BSP support this optimization.
Update #4712.
The __builtin_unreachable() cannot be used with current GCC versions to
tell the compiler that a function does not return to the caller, see:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99151
Add a no return variant of _CPU_Context_switch() to avoid generation of
dead code in _Thread_Start_multitasking() if RTEMS was built with SMP
support enabled.
