Specify FPU for thumv7em-none-eabihf

`-mfpu=fpv4-sp-d16` matches rust specifies for this target.
It is the minimum FPU for this target, which is better than
vfp2 (the default AFAIK).

src/lib.rs

@@ -519,6 +519,10 @@ impl Config {
             }
             if target.starts_with("thumbv7em") {
                 cmd.args.push("-march=armv7e-m".into());
+
+                if target.ends_with("eabihf") {
+                    cmd.args.push("-mfpu=fpv4-sp-d16".into())
+                }
             }
             if target.starts_with("thumbv7m") {
                 cmd.args.push("-march=armv7-m".into());