PowerPC: refactor numdigits

This patches refactor numdigits to remove the _Decimal to interger cast
using assembly and rely on compiler generate code using unions.  This
helps the compiler to generate correct code in debug mode.
libdfp: Correct expected output for quantize tests

This patch fixes the expected output for some tests for _Decimal32 where
the coefficient after rounding/adjustment should overflow and thus the
result should be a NaN value.

Current powerpc code does not handle these cases because it first
convert the _Decimal32 value to _Decimal64 (since the platform does not
have native support for _Decimal32).  The patch also adds a note on
powerpc code to fix these cases.