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 (set `EF_RISCV_RVC`).
*   Set TSO flag from "ztso"
    The "Ztso" extension denotes that the program depends on the stronger
    RVTSO memory ordering (set `EF_RISCV_TSO`).

Author: a4lg

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 {

compiler/rustc_span/src/symbol.rs

@@ -2419,9 +2419,11 @@ symbols! {
         yield_expr,
         ymm_reg,
         yreg,
+        zca,
         zfh,
         zfhmin,
         zmm_reg,
+        ztso,
         // tidy-alphabetical-end
     }
 }