Remove ecall and *ret instructions from riscv::asm

src/asm.rs

@@ -16,9 +16,5 @@ macro_rules! instruction {
 
 
 /// Priviledged ISA Instructions
-instruction!(ecall, "ecall");
 instruction!(ebreak, "ebreak");
-instruction!(uret, "uret");
-instruction!(sret, "sret");
-instruction!(mret, "mret");
 instruction!(wfi, "wfi");