Arm: use is_whitespace()

Arm: 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). At the same time use is_end_of_stmt() instead of an
open-coded nul char check.

In parse_neon_type() be more aggressive and remove the special casing of
certain characters altogether. The original default case simply having
"break" can't have been correct.