Currently, the linker generates one huge DWARF compilation unit for
the entire Go binary. This commit creates a separate compilation unit
and line table per Go package.

We temporarily lose compilation unit PC range information, since it's
now discontiguous, so harder to emit. We'll bring it back in the next
commit.

Beyond being "more traditional", this has various technical
advantages:

* It should speed up line table lookup, since that requires a
  sequential scan of the line table. With this change, a debugger can
  first locate the per-package line table and then scan only that line
  table.

* Once we emit compilation unit PC ranges again, this should also
  speed up various other debugger reverse PC lookups.

* It puts us in a good position to move more DWARF generation into the
  compiler, which could produce at least the CU header, per-function
  line table fragments, and per-function frame unwinding info that the
  linker could simply paste together.

* It will let us record a per-package compiler command-line flags
  (#22168).

Change-Id: Ibac642890984636b3ef1d4b37fe97f4453c2cc84
Reviewed-on: https://go-review.googlesource.com/69973
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>