Add .attributes also for "rv32ec", "rv32em", "rv32emc"

I presume these are required if there happens to be any C or M
instructions in the binary.

At least the "rv32em" variant here is required for our rv32emc if
-Fsingle-hart is disabled.

riscv-rt/src/asm.rs

@@ -35,6 +35,12 @@ cfg_global_asm!(
     ".attribute arch, \"rv32im\"",
     #[cfg(all(riscv32, riscve))]
     ".attribute arch, \"rv32e\"",
+    #[cfg(all(riscv32, riscve, riscvc))]
+    ".attribute arch, \"rv32ec\"",
+    #[cfg(all(riscv32, riscve, riscvm))]
+    ".attribute arch, \"rv32em\"",
+    #[cfg(all(riscv32, riscve, riscvm, riscvc))]
+    ".attribute arch, \"rv32emc\"",
     #[cfg(all(riscv64, riscvm, not(riscvg)))]
     ".attribute arch, \"rv64im\"",
     #[cfg(all(riscv64, riscvg))]