Fix ELF flag for RISC-V targets without explicit ABI

Author: nbdd0121

compiler/rustc_codegen_ssa/src/back/metadata.rs

@@ -283,7 +283,7 @@ pub(crate) fn create_object_file(sess: &Session) -> Option<write::Object<'static
             // Set the appropriate flag based on ABI
             // This needs to match LLVM `RISCVELFStreamer.cpp`
             match &*sess.target.llvm_abiname {
-                "ilp32" | "lp64" => (),
+                "" | "ilp32" | "lp64" => (),
                 "ilp32f" | "lp64f" => e_flags |= elf::EF_RISCV_FLOAT_ABI_SINGLE,
                 "ilp32d" | "lp64d" => e_flags |= elf::EF_RISCV_FLOAT_ABI_DOUBLE,
                 "ilp32e" => e_flags |= elf::EF_RISCV_RVE,