Support characters 128-255 in str_append_ch
Support characters 128-255 in str_append_ch

These were already allowed on platforms where char is unsigned by
default, but not where char is signed.  None of the existing generators
are affected by this, but the new Ada generator is.

Noted by Stephane Carrez in #135.

snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball