cmd/link: clean up dwarf.go

Try to remove the most visible artefacts resulting from the
C to Go translation. It includes:

- refactoring the find function to eliminate goto and variable declarations
- removing useless variables still having a _ = xxx
- decreasing the number of upfront variable declarations

No semantic changes.

Change-Id: I84d981c48b2d9e22e6b9db5f2a703c80c60249ba
Reviewed-on: https://go-review.googlesource.com/15681Reviewed-by: Russ Cox <rsc@golang.org>