rustc_codegen_ssa: More comprehensive RISC-V ELF flags

This commit implements more conformant, more comprehensive RISC-V ELF
flags handling when generating certain object files directly from rustc.

*   Use "zca" instead of "c"
    The "Zca" extension (a subset of "C") is the minimal configuration
    for compressed instructions to set `EF_RISCV_RVC` flag.
*   Set TSO flag from "ztso"
    The "Ztso" extension denotes that the program depends on the RVTSO
    (Total Store Ordering) memory consistency model, which is stronger
    than the standard RVWMO (Weak Memory Ordering) consistency model and
    on ELF targets, we need to set `EF_RISCV_TSO` flag.

compiler/rustc_codegen_ssa/src/back/metadata.rs

@@ -330,11 +330,17 @@ pub(super) fn elf_e_flags(architecture: Architecture, sess: &Session) -> u32 {
             let mut e_flags: u32 = 0x0;
 
             // Check if compression is enabled
-            // `unstable_target_features` is used here because "c" is gated behind riscv_target_feature.
-            if sess.unstable_target_features.contains(&sym::c) {
+            // `unstable_target_features` is used here because "zca" is gated behind riscv_target_feature.
+            if sess.unstable_target_features.contains(&sym::zca) {
                 e_flags |= elf::EF_RISCV_RVC;
             }
 
+            // Check if RVTSO is enabled
+            // `unstable_target_features` is used here because "ztso" is gated behind riscv_target_feature.
+            if sess.unstable_target_features.contains(&sym::ztso) {
+                e_flags |= elf::EF_RISCV_TSO;
+            }
+
             // Set the appropriate flag based on ABI
             // This needs to match LLVM `RISCVELFStreamer.cpp`
             match &*sess.target.llvm_abiname {