Change softload_gnu to be off by default
This involves introducing the concept of opt-in jobs, controlled by the
ENABLE_JOBS environment variable, analogous to the pre-existing
SUPPRESS_JOBS environment variable.
A large bulk of components don't currently fully build with the GNU tools, due to
deficiencies in the shared makefiles and the fact that we don't currently
autobuild ELF versions of libraries for them to link against. Having
softload_gnu be off by default saves us from having to explicitly disable it
in dozens of
.gitlab-ci.yml files only to have to come back later to
re-enable it once the build infrastructure becomes available.