Use const-fn and inline-asm if available

riscv-rt/Cargo.toml

@@ -13,4 +13,5 @@ r0 = "0.2.2"
 riscv = "0.4.0"
 
 [features]
-inline-asm = []
+const-fn = ["riscv/const-fn"]
+inline-asm = ["riscv/inline-asm"]