Add spidermonkey 38 aka mozjs 3.8.

Spidermonkey is mozilla's JavaScript engine. This version is based
on the last release of firefox esr; 38.8.0. This is done because
the stand alone release aren't done often enough. I copied this idea
from the fedora spidermonkey38 spec file.

Probably due to the fact that spidermonkey in the firefox tarball isn't
intended to be installed standalone we need to manualy replace the header
symlinks in the stage dir with the actual header files.

Thanks to miki and streja for build testing on armv6 and aarch64.