[RISCV] Disable atomics for riscv32imc-unknown-none-elf.

src/librustc_target/spec/riscv32imc_unknown_none_elf.rs

@@ -27,7 +27,8 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             linker: Some("rust-lld".to_string()),
             cpu: "generic-rv32".to_string(),
-            max_atomic_width: Some(32),
+            // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86005
+            max_atomic_width: None, //Some(32),
             atomic_cas: false,
             features: "+m,+c".to_string(),
             executables: true,