bugfix: use string keys to set preferred targets (#12921)

Preferred targets were failing because we were looking them up by
Microarchitecture object, not by string.

- [x] Add a call to `str()` to fix target lookup.
- [x] Add a test to exercise this part of concretization.
- [x] Add documentation for setting `target` in `packages.yaml`

lib/spack/docs/basic_usage.rst

@@ -865,9 +865,11 @@ or together by using the reserved keyword ``arch``:
 
    $ spack install libelf arch=cray-CNL10-haswell
 
-Normally users don't have to bother specifying the architecture
-if they are installing software for their current host as in that case the
-values will be detected automatically.
+Normally users don't have to bother specifying the architecture if they
+are installing software for their current host, as in that case the
+values will be detected automatically.  If you need fine-grained control
+over which packages use which targets (or over *all* packages' default
+target), see :ref:`concretization-preferences`.
 
 .. admonition:: Cray machines
 

lib/spack/docs/build_settings.rst

@@ -149,6 +149,7 @@ Here's an example ``packages.yaml`` file that sets preferred packages:
        version: [2.2, 2.4, 2.3]
      all:
        compiler: [gcc@4.4.7, 'gcc@4.6:', intel, clang, pgi]
+       target: [sandybridge]
        providers:
          mpi: [mvapich2, mpich, openmpi]
 
@@ -162,11 +163,11 @@ on the command line if explicitly requested.
 
 Each ``packages.yaml`` file begins with the string ``packages:`` and
 package names are specified on the next level. The special string ``all``
-applies settings to each package. Underneath each package name is
-one or more components: ``compiler``, ``variants``, ``version``,
-or ``providers``.  Each component has an ordered list of spec
-``constraints``, with earlier entries in the list being preferred over
-later entries.
+applies settings to *all* packages. Underneath each package name is one
+or more components: ``compiler``, ``variants``, ``version``,
+``providers``, and ``target``.  Each component has an ordered list of
+spec ``constraints``, with earlier entries in the list being preferred
+over later entries.
 
 Sometimes a package installation may have constraints that forbid
 the first concretization rule, in which case Spack will use the first