Make libunwind provider of unwind virtual pkg

This commit begins addressing the ideas discussed in #8823. The
libunwind library now provides the "unwind" virtual package, which
represents the "libunwind base API" common to LLVM libunwind, Apple's
LLVM libunwind, and non-GNU libunwind.

var/spack/repos/builtin/packages/libunwind/package.py

@@ -36,3 +36,5 @@ class Libunwind(AutotoolsPackage):
 
     version('1.2.1', '06ba9e60d92fd6f55cd9dadb084df19e')
     version('1.1', 'fb4ea2f6fbbe45bf032cd36e586883ce')
+
+    provides('unwind')