Use newer riscv crate

riscv-rt/Cargo.toml

@@ -10,7 +10,7 @@ license = "ISC"
 
 [dependencies]
 r0 = "0.2.2"
-riscv = "0.3.0"
+riscv = "0.4.0"
 
 [features]
 inline-asm = []