[dev.link] cmd/link: don't read symbol name if not needed

[dev.link] cmd/link: don't read symbol name if not needed

Most of the time of goobj2.Sym.Read is spent in reading the
symbol name. Sometimes the name is not needed, so don't read
it.

This brings the linking time pretty much back to the old linker.
On my Mac,

$ time go build cmd/compile     # (new)

real	0m1.016s
user	0m1.164s
sys	0m0.297s

$ time go-tip build cmd/compile # (old)

real	0m0.980s
user	0m1.229s
sys	0m0.309s

Again, this is a bit ugly. Maybe we want to consider moving
symbol names out of Sym, to a separate section of the object
file?

Change-Id: Ia9085c15aa4d614b7eae126e71a79d6806c75055
Reviewed-on: https://go-review.googlesource.com/c/go/+/218480
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>