Fix settings for mpi-serial cases on yellowstone.

Most of the yellowstone-specific Intel settings were set only for
Macros files that were created with `MPILIB="mpich2"`. By changing the
locations where the `MPILIB` attribute is used in `config_compilers`,
this commit makes the settings for mpi-serial cases the same as for
MPICH2 cases, and avoids causing the Macros file to be specific to a
specific MPI library.

Testing:

    None! This might be (in fact seems like it should be) answer-
    changing for mpi-serial builds on yellowstone.

cime_config/cesm/machines/config_compilers.xml

@@ -495,15 +495,15 @@ for mct, etc.
 
 
 
-<compiler MACH="yellowstone" COMPILER="intel" MPILIB="mpich2">
+<compiler MACH="yellowstone" COMPILER="intel">
   <!-- Needed due to the way that netcdf is loaded on yellowstone -->
-  <SCC>$(MPICC)</SCC>
-  <SFC>$(MPIFC)</SFC>
-  <MPICXX>mpiicpc</MPICXX>
+  <SCC MPILIB="mpich2">$(MPICC)</SCC>
+  <SFC MPILIB="mpich2">$(MPIFC)</SFC>
+  <MPICXX MPILIB="mpich2">mpiicpc</MPICXX>
   <ADD_FFLAGS>  -xHost </ADD_FFLAGS>
   <ADD_CFLAGS>  -xHost </ADD_CFLAGS>
   <ADD_CPPDEFS> -DINTEL_MKL -DHAVE_SSE2 </ADD_CPPDEFS>
-  <TRILINOS_PATH>$(TRILINOS_PATH)</TRILINOS_PATH>
+  <TRILINOS_PATH MPILIB="mpich2">$(TRILINOS_PATH)</TRILINOS_PATH>
   <PAPI_INC> /glade/apps/opt/papi/5.3.0/intel/12.1.5/include/</PAPI_INC>
   <PAPI_LIB>/glade/apps/opt/papi/5.3.0/intel/12.1.5/lib64 </PAPI_LIB>
   <ADD_SLIBS> -Wl,-rpath ${PAPI_LIB} -L${PAPI_LIB} -lpapi</ADD_SLIBS>