Add target_has_atomic* symbols if any atomic width is supported

Atomic operations for different widths (8-bit, 16-bit, 32-bit etc.) are
guarded by `target_has_atomic = "value"` symbol (i.e. `target_has_atomic
= "8"`) (and the other derivatives), but before this change, there was
no width-agnostic symbol indicating a general availability of atomic
operations.

This change introduces:

* `target_has_atomic_load_store` symbol when atomics for any integer
  width are supported by the target.
* `target_has_atomic` symbol when also CAS is supported.

Fixes rust-lang#106845

Signed-off-by: Michal Rostecki <vadorovsky@gmail.com>

Author: vadorovsky

compiler/rustc_session/src/config.rs

@@ -957,6 +957,7 @@ fn default_configuration(sess: &Session) -> CrateConfig {
     if sess.target.has_thread_local {
         ret.insert((sym::target_thread_local, None));
     }
+    let mut has_atomic = false;
     for (i, align) in [
         (8, layout.i8_align.abi),
         (16, layout.i16_align.abi),
@@ -965,6 +966,7 @@ fn default_configuration(sess: &Session) -> CrateConfig {
         (128, layout.i128_align.abi),
     ] {
         if i >= min_atomic_width && i <= max_atomic_width {
+            has_atomic = true;
             let mut insert_atomic = |s, align: Align| {
                 ret.insert((sym::target_has_atomic_load_store, Some(Symbol::intern(s))));
                 if atomic_cas {
@@ -981,6 +983,12 @@ fn default_configuration(sess: &Session) -> CrateConfig {
             }
         }
     }
+    if sess.is_nightly_build() && has_atomic {
+        ret.insert((sym::target_has_atomic_load_store, None));
+        if atomic_cas {
+            ret.insert((sym::target_has_atomic, None));
+        }
+    }
 
     let panic_strategy = sess.panic_strategy();
     ret.insert((sym::panic, Some(panic_strategy.desc_symbol())));