Investigate removing fallback logic for Solaris/Illumos

[josephlr]

I filed rust-lang/rust#124549 to see what the minimum version of Illumos/Solaris is for Rust. If we get clarification on this, we can remove the file-based fallback code for these targets.

[josephlr]

From the looks of that discussion, we should be good to remove the fallback code for Solaris and Illumos