Implement reloc_names() for all targets.

This gets rid of the last TargetIsa method with a default
implementation.

lib/cretonne/src/isa/arm32/binemit.rs

@@ -4,3 +4,5 @@ use binemit::{CodeSink, bad_encoding};
 use ir::{Function, Inst};
 
 include!(concat!(env!("OUT_DIR"), "/binemit-arm32.rs"));
+
+pub static RELOC_NAMES: [&'static str; 1] = ["Call"];