Fixes for PowerPC

[barracuda156]

Description

There are trivial fixes which do not introduce any new code or functionally modify existing one.
What this does is:

1. Fix PowerPC macros so that they also work on Darwin.
2. Unbreak assembler parts on Darwin, where registers must be prefixed, unlike ELF ABI.

P. S. While it is possible to simply add register prefices via a new macro (which will expand to nothing on Linux/BSD) instead of a separate implementation, I think this way is preferable, since it will prevent Darwin ABI from being accidentally broken when someone updates this later on, adding VSX instructions, for example, or crypto of later ISA. It also matches existing code style, which, for example, keeps ppc32 and ppc64 in separate chunks, not mixing them.

[wolfSSL-Bot]

Can one of the admins verify this patch?

[barracuda156]

@dgarske Since you have changed that code in asm.c in a2ed222 earlier, did you have anything else in mind besides macOS? AFAIK, no one else uses prefixed registers, but if something does, I need to add a macro for that case as well.

@JacobBarthelmeh (Tagging you in relation to the earlier commit.)

[dgarske]

Hi @barracuda156, thanks for your PR's. In order to accept a third party contribution we need a signed agreement. Please email support at wolfssl dot com to get that process started.
Thank you, David Garske, wolfSSL

[barracuda156]

@dgarske Done

[anhu]

Contributor agreement has been approved. We're good to move forward on this.