simplify outline atomics

 * Rely on libSystem when targeting macOS.
 * Make tools/gen_outline_atomics.zig more idiomatic.
 * Remove the CPU detection / auxval checking from compiler_rt. This
   functionality belongs in a different component. Zig's compiler_rt
   must not rely on constructors. Instead it will export a symbol for
   setting the value, and start code can detect and activate it.
 * Remove the separate logic for inline assembly when the target does or
   does not have lse support. `.inst` works in both cases.

Author: andrewrk

lib/compiler_rt.zig

@@ -1,8 +1,14 @@
+const builtin = @import("builtin");
+
 pub const panic = @import("compiler_rt/common.zig").panic;
 
 comptime {
     _ = @import("compiler_rt/atomics.zig");
-    _ = @import("compiler_rt/lse_atomics.zig");
+
+    // macOS has these functions inside libSystem.
+    if (builtin.cpu.arch.isAARCH64() and builtin.os.tag != .macos) {
+        _ = @import("compiler_rt/lse_atomics.zig");
+    }
 
     _ = @import("compiler_rt/addf3.zig");
     _ = @import("compiler_rt/addhf3.zig");