PPC: Allow partial fills in writeNopData()

When asked to pad an irregular number of bytes, we should fill with
zeros.  This is consistent with the behavior specified in the AIX
Assembler Language Reference as well as other LLVM and binutils
assemblers.

N.B. There is a small deviation from binutils' PPC assembler:
when handling pads which are greater than 4 bytes but not mod 4,
binutils will not emit any NOP sequences at all and only use zeros.
This may or may not be a bug but there is no excellent rationale as to
why that behavior is important to emulate.  If that behavior is needed,
we can change writeNopData() to behave in the same way.

This fixes PR17352.