Make `mkComment` work for all architectures

The Instruction Type Class' mkComment function should create an ASM comment. But the current implementations for x86, PPC and SPARC look like this:

mkComment = const []

Only AARCH64 implements this. Most likely because, e.g., x86 COMMENT takes a FastString (and AARCH64's an SDoc). In turn, this may be because CmmComment has a FastString. Anyway, AARCH64 just converts this in CodeGen.hs.

Motivation

Adding comments to generated assembly can be very useful when debugging said assembly.

Proposal

I'd change x86, PPC and SPARC to work exactly like AARCH64, that is, change COMMENT to take an SDoc and convert from FastString to SDoc.

If there are no objections to this, I'd just create an MR.

(@angerman what do you say?)