powerpc: Fix operand prefixes

The file sysdeps/powerpc/sysdeps.h defines aliases for condition register
operands.  E.g.: 'cr7' means condition register 7.  On the one hand, this
increases readability, as it makes it easier for readers to know whether the
operand is a condition register, a general purpose register or an immediate.
On the other hand, this permits that condition registers be written as if they
were general purpose, and vice-versa, thus reducing the readability of the
code.

This commit removes some of these unintentional misuses.

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