support riscv64 target

Signed-off-by: xermicus <cyrill@parity.io>

crates/linker/Cargo.toml

@@ -5,6 +5,9 @@ edition = "2021"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
+[features]
+riscv-64 = []
+
 [dependencies]
 inkwell = { workspace = true }
 tempfile = { workspace = true }