Setting environment variables when doing remove execution

[Tobias Burnus]

Hi all,

the following is motivated by GCC testsuite – which uses 
'dg-set-target-env-var' to set per-test-case environment variables (used 
with the address-sanitizer (ASAN) and in the testsuite for 
OpenMP/OpenACC (libgomp)). [The latter also uses 'setenv' for offloading 
for all runs per offloading target.] – GCC uses 'setenv' for this 
purpose, which works well if host = target (= ![is_remote…]) – but when 
doing remote execution, it does not work.

The question is how to best support setting the environment variable 
remote. I could imagine to either pass environment variables alongside 
when running remote_spawn/remote_exec or to have a remove_setenv.

Thoughts?

Cheers,

Tobias