When building golang, the environment variable GOROOT_FINAL can be set
to indicate a different installation location from the build
location. This works fine, except that the goc2c build step embeds
line numbers in the resulting c source files that refer to the build
location, no the install location.

This would not be a big deal, except that in turn the linker uses the
location of runtime/string.goc to embed the gdb script in the
resulting binary and as a net result, the debugger now complains that
the script is outside its load path (it has the install location
configured).

See https://code.google.com/p/go/issues/detail?id=8524 for the full
description.

Fixes #8524.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/128230046