ci: Add option to use different version of LLVM
Some targets require different version of LLVM, so now it's possible to set it in the target's environment. Mind that the highest available version depends on the base Debian image.
The change bumps LLVM version for all Linux targets from 14 to 16, and from 16 to 19 for RISC-V. Windows targets stay the same, as they use a different source of LLVM (MinGW-compatible, which is currently version 18).
Signed-off-by: Marek Pikuła m.pikula@partner.samsung.com