bsps/powerpc: Use System V small-data area

The 32-bit ABIs for thread-local storage (TLS) and EABI read-only
small-data area have a conflicting register (r2) usage.  Use the System
V small-data instead (here r2 is system reserved).
bsps: Add empty cache manager

This is necessary to add tests that use the cache manager.  For example
to get better estimates of worst-case timings.
sptests/spcpucounter01: Adjust test

Adjust test to work with clock driver based CPU counters.  They have a
period equal to the clock tick interval.
score: Fix thread TLS area initialization

Do not use _TLS_Size here since this will lead GCC to assume that this
symbol is not 0 and the later > 0 test will be optimized away.