debug/gosym: use "go build" instead of hand-running asm and link

Currently, TestPCLine manually invokes asm and link on its test data.
Once we introduce symbol ABIs this is going to become problematic
because the test program defines main.main and main.init in assembly
so they use ABI0, but the runtime expects to find them with the
internal ABI.

There are various ways we could solve this. This CL moves main.main
and main.init into Go code and switches to using "go build" to compile
and link the test binary. This has the added advantage of simplifying
this test.

For #27539.

Change-Id: I4c0cf6467f7a39e6b1500eca6ad2620b5ef2b73c
Reviewed-on: https://go-review.googlesource.com/c/146857


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>