Indicate that ppc-linux supports alien-callbacks.

It does for a long time, but the feature wasn't pushed, and some tests
weren't executed, but they do pass.
Reduce profiling interval in sb-sprof tests.

Due to ARM being slower, it can't handle constant sigprof signals. A
better sigprof handle, which does less work, would solve it.
Save FP registers during call_into_lisp on ARM.

d8-d15 have to be saved by the callee.
Flush icache in alien-callback-assembler-wrapper on ARM.