[dev.link] cmd/link: stream out external relocations on AMD64 ELF

[dev.link] cmd/link: stream out external relocations on AMD64 ELF

Currently, when external linking, in relocsym (in asmb pass), we
convert Go relocations to an in-memory representation of external
relocations, and then in asmb2 pass we write them out to the
output file. This is not memory efficient.

This CL makes it not do the conversion but directly stream out
the external relocations based on Go relocations. Currently only
do this on AMD64 ELF systems.

This reduces memory usage, but makes the asmb2 pass a little
slower.

Linking cmd/compile with external linking:

name             old time/op    new time/op    delta
Asmb_GC            83.8ms ± 7%    70.4ms ± 4%  -16.03%  (p=0.008 n=5+5)
Asmb2_GC           95.6ms ± 4%   118.2ms ± 5%  +23.65%  (p=0.008 n=5+5)
TotalTime_GC        1.59s ± 2%     1.62s ± 1%     ~     (p=0.151 n=5+5)

name             old alloc/op   new alloc/op   delta
Asmb_GC            26.0MB ± 0%     4.1MB ± 0%  -84.15%  (p=0.008 n=5+5)
Asmb2_GC           8.19MB ± 0%    8.18MB ± 0%     ~     (p=0.222 n=5+5)

name             old live-B     new live-B     delta
Asmb_GC             49.2M ± 0%     27.4M ± 0%  -44.38%  (p=0.008 n=5+5)
Asmb2_GC            51.5M ± 0%     29.7M ± 0%  -42.33%  (p=0.008 n=5+5)

TODO: figure out what is slow. Possible improvements:
- Remove redundant work in relocsym.
- Maybe there is a better representation for external relocations
  now.
- Fine-grained parallelism in emitting external relocations.
- The old elfrelocsect only iterates over external relocations,
  now we iterate over all relocations. Is it too many?

Change-Id: Ib0a8ee8c88d65864c62b89a8d634614f7f2c813e
Reviewed-on: https://go-review.googlesource.com/c/go/+/242603
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>