darwin packages.yaml defaults: add unwind provider

This commit adds default unwind providers to the default packages.yaml
for darwin. Compiler versions are supplied with the apple-unwind
package so that apple-unwind is only used with Apple's clang fork, and
not with LLVM's clang.

etc/spack/defaults/darwin/packages.yaml

@@ -18,3 +18,10 @@ packages:
     compiler: [clang, gcc, intel]
     providers:
       elf: [libelf]
+      unwind: [apple-libunwind]
+  apple-libunwind:
+    paths:
+    # Apple bundles libunwind version 35.3 with macOS 10.9 and later,
+    # although the version number used here isn't critical
+      apple-libunwind@35.3: /usr
+    buildable: False