C54x: use is_end_of_stmt()

C54x: use is_end_of_stmt()

... instead of open-coding it.

In tic54x_stringer() this also fixes an array overrun issue: Converting
plain char to unsigned int could have yielded huge values when plain
char is a signed type.

In subsym_substitute() also convert a local variable to plain char, as
that's what it's really holding (and how it's used everywhere else).