Improve PoCL portability

[maleadt]

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.

[giordano]

The gcc ones are in compilersupportlibraries, but we don't have the libc ones anywhere (well, in the compiler shards used by BinaryBuilder, but not easily accessible by anywhere else).

[maleadt]

Maybe I ought to copy stuff over from the sysroot when creating pocl_jll then?

[giordano]

Uhm, that could work, as long as you don't add them as library products which are automatically dlopened (file products are fine, if you do want to add them, but not necessary)

[maleadt]

Note to self: PoCL is broken on macOS right now because of the switch to lld:

      From worker 28:	ld64.lld: error: must specify -platform_version
      From worker 28:	error: linker command failed with exit code 1 (use -v to see invocation)
      From worker 28:	Final linking of kernel _Z24partial_mapreduce_device8identity1_5Int163ValILi10EE16CartesianIndicesILi1E5TupleI5OneToI5Int64EEESA_13CLDeviceArrayIS1_Li2ELi1EESB_IS1_Li1ELi1EE failed.