reduce memory use of pbchunk conversion (#820)

C code generated by the pbchunk conversion is especially verbose, with
long macro names for simple operations and lots of comments to reflect
source instructions. Generation of that code was formerly rendering to
text in memory, and then splitting into the output files, which turns
out to use a lot of memory. Converting to text at the last minute can
have a much lower peak memory use.

If memory is at a premimum, then rendering to a temporary file would
work even better, but this change already brings memory use in line
with other build actions without having to deal with a temporary file.
Memory use could also be reduced by dropping comments that connect to
source instructions; again, though, doesn't seem necessary for now.

Author: mflatt