gitlab-ci.yml: Allow custom # of parallel linkers

Define a new variable LD_JOBS, that can be used to select
the maximum number of linking jobs to be executed in parallel.
If the variable is not defined, maintain the default given by
make -j

Currently, make parallelism at build time is based on the number
of cpus available.

This doesn't work well with LTO at linking, because with LTO the
linker has to load in memory all the intermediate object files
for optimization.
The end result is that, if the gitlab runner happens to run two
linking processes at the same time, the job will fail with an
out-of-memory error,

This patch leverages the ability to maintain high parallelism at
compile time, but limit the number of linkers executed in parallel.

Signed-off-by: Daniele Buono <dbuono@linux.vnet.ibm.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210304030948.9367-2-dbuono@linux.vnet.ibm.com>
Message-Id: <20210305092328.31792-8-alex.bennee@linaro.org>