Add BPF target

This change adds the bpfel-unknown-none and bpfeb-unknown-none targets
which can be used to generate little endian and big endian BPF

compiler/rustc_codegen_cranelift/src/toolchain.rs

@@ -67,6 +67,7 @@ fn linker_and_flavor(sess: &Session) -> (PathBuf, LinkerFlavor) {
                     LinkerFlavor::Msvc => "link.exe",
                     LinkerFlavor::Lld(_) => "lld",
                     LinkerFlavor::PtxLinker => "rust-ptx-linker",
+                    LinkerFlavor::BpfLinker => "bpf-linker",
                 }),
                 flavor,
             )),