[Runner] Work around wrong path of compiler libraries for riscv64. Again

[giordano]

I don't know how this has happened, because I thought this was resolved by JuliaPackaging/Yggdrasil@123ac9b (#10132) (and the path is right for x86_64, aarch64, and powerpc64le), but the compiler support libraries ended up again in lib/ instead of lib64/ for riscv64, so we have to repeat #402. I thought we had a test about this (create a C++ library, and link to it with cc), apparently we don't, probably we should, but I don't have the time to add it right now.

[giordano]

Ok, I believe this should be working now, tests were failing before because I was testing clang as well: we're passing -L/opt/${target}/${target}/${lib64} -Wl,-rpath-link,/opt/${target}/${target}/${lib64} only to gcc, we may want to do it for clang too, but leaving that change for the future for the time being.