Avoid generating unnecessary NIF functions with -strbin

Avoid generating utf8_to_uint32 as well as utf8_count_codepoints if
the -strbin is specified with -nif.

Previously, the utf8_to_uint32 was always generated.

The utf8_count_codepoints, too, was always generated, and used to
count code-points, the value then thrown away, for valid utf-8.
Prepare for 3.17.5