Allow to use a custom dsymutil command

[glandium]

src/librustc_codegen_llvm/back/link.rs does this:

match Command::new("dsymutil").arg(out_filename).output() {

dsymutil might not be in your PATH, or worse, have a different name (like, llvm-dsymutil)

[glandium]

Cc: @alexcrichton

[alexcrichton]

Seems plausible to me! We can likely stabilize this as -C dsymutil=... like we have -C linker=...

[wesleywiser]

Visited during wg-debugging's triage meeting. It looks like this is still the case

rust/compiler/rustc_codegen_ssa/src/back/link.rs

Line 989 in 4c56655

let prog = Command::new("dsymutil").arg(out_filename).output();