PowerPC: Fix operand prefixes

The file sysdeps/powerpc/sysdeps.h defines aliases for register operands,
which add the letter 'r' as a prefix to a register name.  E.g.: register 20
can be written as 'r20', instead of '20'.  On the one hand, this increases
readability, as it makes it easier for readers to know whether the operand is a
register or an immediate.  On the other hand, this permits that immediate
operands be written as if they were registers, and vice-versa, thus reducing
the readability of the code.

This commit removes some of these unintentional misuses.

This commit also increases readability of the code by adding the prefix 'cr' to
some uses of the control register.

Both changes have no effect on the final code.  Checked with objdump.

	* sysdeps/powerpc/powerpc64/power8/strncpy.S: Remove or add register
	prefix from operands.