Cleanup and comment improvements

[taiki-e]

• Add per-line pseudo code style code comments to almost all inline assemblies.
  Hopefully this will make the code easier to understand for people who are not familiar with assembly.
  For example:
  

  atomic-maybe-uninit/src/arch/powerpc.rs

  Lines 187 to 192 in d901983

  	$release, // fence
  	"2:", // 'retry:
  	concat!("l", $suffix, "arx {out}, 0, {dst}"), // atomic { RESERVE = (dst, size_of($int_type)); out = *dst }
  	concat!("st", $suffix, "cx. {val}, 0, {dst}"), // atomic { if RESERVE == (dst, size_of($int_type)) { *dst = val; cr0.EQ = 1 } else { cr0.EQ = 0 }; RESERVE = None }
  	"bne %cr0, 2b", // if cr0.EQ == 0 { jump 'retry }
  	$acquire, // fence

• Add module-level comments to some architectures describing the types of atomic instructions provided by that architecture. I plan to add this for other architectures in the future.

  UPDATE: this now extended to more architectures as "Atomic operation overview by architecture". See the linked docs for more.

• Clean up suboptimal or redundant code found during the above process.

• Add links to generated assemblies for tier 3 targets.