Update llvm from 7.0.0-1 to 8.0.0 (#3248)

* Update llvm from 7.0.0-1 to 8.0.0

Add more build dependencies to llvm

Use gold linker

Create architecture independent wrapper scripts

Remove -DLLVM_TARGETS_TO_BUILD flag and simplify build logic

Add pre-built binaries

* Abstract version in wrapper scripts