Moxie: use is_whitespace()

Moxie: 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 ISSPACE() uses. At the same time use
is_end_of_stmt() instead of an open-coded check in adjacent code. While
at it also drop a redundant whitespace skipping loop.