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".

Change-Id: I0ef3c44873b3d788efc14dc7cbfc37df033bf9ed
Reviewed-on: https://go-review.googlesource.com/85955
Reviewed-by: Ian Lance Taylor <iant@golang.org>
3 files changed