v850: use is_whitespace()

v850: use is_whitespace()

Wherever blanks are permissible in input, tabs ought to be permissible,
too. This is particularly relevant when -f is passed to gas (alongside
appropriate input). Also convert open-coded checks as well as ISSPACE()
uses. At the same time use is_end_of_stmt() instead of a kind-of-open-
coded check in adjacent code.