cmd/link: replace custom hashmap in DWARF writer

cmd/link: replace custom hashmap in DWARF writer

Also stop creating a map for each symbol, as it does not seem to help.

Linking juju:
	tip:  real 0m5.470s user 0m6.131s
	this: real 0m4.811s user 0m5.582s

Change-Id: Ib3d931c996396a00942581770ff32df1eb8d6615
Reviewed-on: https://go-review.googlesource.com/20140
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>