Add gcc, gfortran and clang on Windows and Ubuntu images

[ivan-nosar]

This PR adds a bunch of software tools on VM images:

• gcc
  • Ubuntu: gcc-7 was installed on image previously. gcc-8 and gcc-9 added.
  • Windows: gcc-8 was installed on image previously in scope of MinGW tool. No new version added.
• gfortran
  • Ubuntu: gfortran-8 and gfortran-9 added
  • Windows: gfortran-8 was installed on image previously in scope of MinGW tool. No new version added.
• Clang
  • Ubuntu: clang-6 was installed on image previously. clang-8 and clang-9 was added (along with llvm-8 and llvm-9)
  • Windows: clang-9 was added along with llvm-9

[q4a]

Why did you remove clang-9 and llvm-9 from Windows?
I'm talking about this commit:
9c06dae

[EwoutH]

@ivan-nosar Could you add GCC 10 to the Ubuntu 18.04 image? It's now available on Debian testing so it can be installed with apt.

[maxim-lobanov]

@EwoutH , feel free to create the issue for that.

[autodidaddict]

Why did you remove clang-9 and llvm-9 from Windows?
I'm talking about this commit:
9c06dae

Came here to find out the same thing. I've got a windows build failing because clang is missing.

[advancedwebdeveloper]

Is it possible to provide https://github.com/llvm/llvm-project/releases/tag/llvmorg-11.0.0 ?
Clang 11, lld and libc++ of the latest stable version, I mean.

[miketimofeev]

@advancedwebdeveloper could you please create an issue with this request?