ENH: Adding numeric_traits specializations for "long long"

This patch applies commits r34558 and r36873 from VXL upstream
authored by Brad King.

Change-Id: Ie233754645f3d4fc434578b5eb0871d3dd74c860
ENH: instantiate vnl types with "long long" on all platforms

This works toward portable instantiation of images with int64_t
types.

Change-Id: Id228cbd38794336898b0b0df06cf9524641d01b0
Merge topic 'AddVnlNumericTraits_long_long'

02db47b ENH: instantiate vnl types with "long long" on all platforms
35cd28d ENH: Adding numeric_traits specializations for "long long"