cmd/compile: update object file format for DWARF file table

In CL 188317, we generate the debug_lines in the compiler, and created a
new symbol to hold the line table. Here we modify the object file format
to output the file table.

Change-Id: Ibee192e80b86ff6af36467a0b1c26ee747dfee37
Reviewed-on: https://go-review.googlesource.com/c/go/+/191167


Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>