[release-branch.go1.15] cmd/{compile,link}: backport fix for issue 39757

During Go 1.15 development, a fix was added to the toolchain for issue
information. The 1.15 line tables were slightly malformed in the way
that they used the DWARF "end sequence" operator, resulting in
incorrect line table info for the final instruction in the final
function of a compilation unit.

This problem was fixed in https://golang.org/cl/235739, which made it
into Go 1.15. It now appears that while the fix works OK for linux, in
certain cases it causes issues with the Darwin linker (the "address
not in any section" ld64 error reported in issue #40974).

During Go 1.16 development, the fix in https://golang.org/cl/235739
was revised so as to fix another related problem (described in issue #39757);
the newer fix does not trigger the problem in the Darwin linker however.

This CL back-ports the changes in https://golang.org/cl/239286 to the
1.15 release branch, so as to fix the Darwin linker error.

Updates #38192.
Updates #39757.
Fixes #40974.

Change-Id: I9350fec4503cd3a76b97aaea0d8aed1511662e29
Reviewed-on: https://go-review.googlesource.com/c/go/+/258422


Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Trust: Than McIntosh <thanm@google.com>