[dev.link] cmd/link: remove buffered file I/O from OutBuf
Recreation of CL 228317.
The problem with that original CL was a late requested change,
reordering reloc and asmb, resulting in symbols having stale pointers to
their data. I've fixed this by preallocating the heap variable in OutBuf
for platforms w/o mmap.
Change-Id: Icdb392ac2c8d6518830f4c84cf422e78b8ab68c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/228782
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Than McIntosh <thanm@google.com>