make -j4 for LLVM requires about 36GB (I know because I tried with 32GB). 16 gb is almost enough to maybe build a debug build of LLVM without threading, but it will take a few hours on a chip that slow. With only 8gb, you are going to be absolutely trashing your swap (which at pcie gen 1x2 speeds will be pretty darn torturous). I'm guessing this system will take ~8 hours to build LLVM.
Fortunately the LLVM build framework allows you to specify how many link steps can be done in parallel separately from the number of other things done in parallel.
Also, linkers other than GNU `ld` (or `gold` which is faster but used more RAM) use a lot less RAM e.g. `mold`, or even better LLVM's own `lld`.
