[feat] Add support for UPC language and launchers

[sleak-lbl]

testing functionality of UPC and UPC++ code requires reframe to know how to compile and run UPC code. This PR adds those features

[jenkins-cscs]

Can I test this patch?

[codecov-commenter]

Codecov Report

Merging #1333 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1333   +/-   ##
=======================================
  Coverage   91.77%   91.78%           
=======================================
  Files          83       83           
  Lines       12523    12561   +38     
=======================================
+ Hits        11493    11529   +36     
- Misses       1030     1032    +2     

Impacted Files	Coverage Δ
reframe/core/buildsystems.py	96.75% <100.00%> (ø)
reframe/core/launchers/mpi.py	98.90% <100.00%> (+0.27%)	⬆️
unittests/test_launchers.py	94.93% <100.00%> (+0.73%)	⬆️
reframe/core/pipeline.py	92.84% <0.00%> (-0.38%)	⬇️

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[vkarak]

@sleak-lbl We will also need an entry about these launchers in documentation here.

[vkarak]

@sleak-lbl I think something got wrong with your updating this PR. It seems that you merged with your own modified master and pushed here. I'll try to fix it.

[vkarak]

I forced push to this PR and fixed the commits. It seems that you have rebased locally over master and then you pushed here doing essentially a merge.

[vkarak]

test this please

[sleak-lbl]

oops, I was trying to rebase to the eth-cscs master, must have used the wrong one .. thanks for fixing