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.