cmd/compile: give TLS relocations a name when dumping assembly

cmd/compile: give TLS relocations a name when dumping assembly

Before:

	...
	0x00d0 ff ff ff e8 00 00 00 00 e9 23 ff ff ff cc cc cc  .........#......
	rel 5+4 t=14 +0
	rel 82+4 t=13 runtime.writeBarrier+0
	...

After:

	...
	0x00d0 ff ff ff e8 00 00 00 00 e9 23 ff ff ff cc cc cc  .........#......
	rel 5+4 t=14 TLS+0
	rel 82+4 t=13 runtime.writeBarrier+0
	...

Change-Id: Ibdaf694581b5fd5fb87fa8ce6a792f3eb4493622
Reviewed-on: https://go-review.googlesource.com/21545
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>