Redefined BLIS_NUM_ARCHS to update automatically.

Details:
- Changed BLIS_NUM_ARCHS from a cpp macro definition to the last enum
  value in the arch_t enum. This means that it no longer needs to get
  updated manually whenever new subconfigurations are added to BLIS.
  Also removed the explicit initial index assigment of 0 from the
  first enum value, which was unnecessary due to how the C language
  standard mandates indexing of enum values. Thanks to Devin Matthews
  for originally submitting this as a PR in #446.
- Updated docs/ConfigurationHowTo.md to reflect the aforementioned
  change.

docs/ConfigurationHowTo.md

@@ -677,14 +677,14 @@ Adding support for a new-subconfiguration to BLIS is similar to adding support f
           BLIS_ARCH_POWER7,
           BLIS_ARCH_BGQ,
 
-          BLIS_ARCH_GENERIC
+          BLIS_ARCH_GENERIC,
+
+          BLIS_NUM_ARCHS
 
       } arch_t;
       ```
-      Additionally, you'll need to update the definition of `BLIS_NUM_ARCHS` to reflect the new total number of enumerated `arch_t` values:
-      ```c
-      #define BLIS_NUM_ARCHS 16
-      ```
+      Notice that the total number of `arch_t` values, `BLIS_NUM_ARCHS`, is updated automatically.
+
 
 
    * **`frame/base/bli_gks.c`**. We must also update the global kernel structure, or gks, to register the new sub-configuration during library initialization. Sub-configuration registration occurs in `bli_gks_init()`. For `knl`, updating this function amounts to inserting the following lines