Move the list of libgo, gotool, and check-target packages into
separate files, then read the file contents as part of the build
process on the fly. This is intended to enable other build tooling to
share the canonical list of target packages (avoid duplication).
Reviewed-on: https://go-review.googlesource.com/89515
libgo: revise rules for runtime.inc generation
Refactor code for generating runtime.inc: extract out the relevant
commands and place them in a separate shell script ("mkruntimeinc.sh").
Update rules to avoid generating macros whose names begin with "$",
such as "#define $sinkconst0 0".
Reviewed-on: https://go-review.googlesource.com/85955
From-SVN: r259863
