Improve PoCL portability

PoCL compiles using Clang, which assumes that the host OS provides certain libraries. For example, on Linux:

```
From worker 6: ld.lld: error: unable to find library -lm
From worker 6: ld.lld: error: unable to find library -lgcc
From worker 6: ld.lld: error: unable to find library -lgcc_s
From worker 6: ld.lld: error: unable to find library -lc
From worker 6: ld.lld: error: unable to find library -lgcc
From worker 6: ld.lld: error: unable to find library -lgcc_s
```

... or, on macOS:

```
From worker 2: ld64.lld: error: library not found for -lm
From worker 2: ld64.lld: error: library not found for -lm
From worker 2: ld64.lld: error: library not found for -lSystem
```

This is obviously not something we can work with, as we want our artifacts to be stand-alone.

@giordano Do you know where I could get these from, or what is the approach taken on Yggdrasil? CSL_jll seems to only provide `libgcc_s`.