score: Delete SYSTEM_STATE_BEGIN_MULTITASKING

Nothing happened between the SYSTEM_STATE_BEGIN_MULTITASKING to
SYSTEM_STATE_UP transition.
smp: Generalize _Thread_Start_multitasking()

Add context parameter to _Thread_Start_multitasking() and use this
function in rtems_smp_secondary_cpu_initialize().  This avoids
duplication of code.

Fix missing floating point context initialization in
rtems_smp_secondary_cpu_initialize().  Now performed via
_Thread_Start_multitasking().
smptests/smpmigration01: New test
smptests/smpswitchextension01: New test
sptests/sp37: Add ISR set/get level tests
arm: Fix CPU_MODES_INTERRUPT_MASK

The set of interrupt levels must be a continuous range of non-negative
integers starting at zero.