[CI] Add oneAPI compilers (Intel/LLVM) to testing matrix.

[kris-rowe]

Description

• Adds the oneAPI compilers to the testing matrix used by GitHub.
• The context added runs on the latest ubuntu using the CMake build.
• Compilers are retrieved using apt.
• Enables CPU testing of the OpenMP, OpenCL, and DPC++ modes
• Adds the OpenCL and DPC++ modes to codecov

[codecov]

Codecov Report

Merging #542 (72065ab) into main (2abc3e9) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #542      +/-   ##
==========================================
- Coverage   76.36%   76.35%   -0.01%     
==========================================
  Files         262      262              
  Lines       19523    19521       -2     
==========================================
- Hits        14908    14906       -2     
  Misses       4615     4615              

Impacted Files	Coverage Δ
src/occa/internal/bin/occa.cpp	37.20% <100.00%> (ø)
src/occa/internal/lang/modes/dpcpp.cpp	80.37% <100.00%> (ø)
src/occa/internal/utils/sys.cpp	76.51% <0.00%> (-0.11%)	⬇️
src/occa/internal/lang/specialMacros.cpp	60.92% <0.00%> (ø)

[SFrijters]

Just an additional note: I've recently started preparing our in-house libraries for IntelLLVM as well and one of the issues I ran into is that CMake doesn't properly detect the compiler before 3.20 (https://cmake.org/cmake/help/latest/release/3.20.html#compilers). I've added a compatibility workaround to our CMake, maybe that could be useful for occa as well if occa needs compiler-specific flags at some point.