scons: Fix clang version identification for OSX
scons: Fix clang version identification for OSX

The version string may have additional trailing information
(transplanted from b7e7533097b97aa7cad4c008767a54077717932a)

andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin