cmd/internal/obj, cmd/link: put all symbol data in one contiguous section

cmd/internal/obj, cmd/link: put all symbol data in one contiguous section

Another object file change, gives a reasonable improvement:

name       old s/op   new s/op   delta
LinkCmdGo  0.46 ± 3%  0.44 ± 9%  -3.34%  (p=0.000 n=98+82)
LinkJuju   4.09 ± 4%  3.92 ± 5%  -4.30%  (p=0.000 n=98+99)

I guess the data section could be mmap-ed instead of read, I haven't tried
that.

Change-Id: I959eee470a05526ab1579e3f5d3ede41c16c954f
Reviewed-on: https://go-review.googlesource.com/20928
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>