gas MMIX: Use more of is_... framework like is_whitespace and is_end_of_stmt

gas MMIX: Use more of is_... framework like is_whitespace and is_end_of_stmt

Convert uses of ISSPACE() and testing for specific characters into
calls to is_whitespace and is_end_of_stmt.  While doing that, also
remove some redundant tests, like ';' together with is_end_of_line[]
and is_whitespace and !is_end_of_line.

Note the invalid casts being fixed as part of moving to is_... macros;
there were (unsigned int) where there should have been (unsigned char)
applied on char as index to is_end_of_line[].

Beware that the input language changes slightly: some constructs with
whitespace characters other than space and TAB are now invalid.