This per-symbol table was written with the strategy:

	1. record offset and write fake header
	2. write body
	3. seek back to fake header
	4. write real header

This CL collects the per-symbol body into a []byte, then writes the
real header followed by the body to the output file. This saves two
seeks per-symbol and overwriting the fake header.

Small performance improvement (3.5%) in best-of-ten links of godoc:

tip:  real 0m1.132s user 0m1.256s
this: real 0m1.090s user 0m1.210s

I'm not sure if the performance measured here alone justifies it,
but I think this is an easier to read style of code.

Change-Id: I1663901eb7c2ee330591b8b6550cdff0402ed5dc
Reviewed-on: https://go-review.googlesource.com/20074Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>