build: Make genlast write the "split" files

This gets a further speedup of about 2s (12s -> 9.5s elapsed build time)
for stm32f405_feather

For what are probably historical reasons, the qstr process involves
preprocessing a large number of source files into a single "qstr.i.last"
file, then reading this and splitting it into one "qstr" file for each
original source ("*.c") file.

By eliminating the step of writing qstr.i.last as well as making the
regular-expression-matching part be parallelized, build speed is further
improved.

Because the step to build QSTR_DEFS_COLLECTED does not access
qstr.i.last, the path is replaced with "-" in the Makefile.

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