[PowerPC64] Hide dssall instruction from llvm assembler

When doing a build for a modern CPUTYPE, llvm will throw errors if obsolete
instructions are used, even if they will never run due to runtime checks.

Hiding the dssall instruction from the assembler fixes kernel build when
overriding CPUTYPE, without having any effect on the generated binary.

This has been in my local tree for over a year and is well tested across
a variety of machines.

Sponsored by: Tag1 Consulting, Inc.