Redo the Unix source code generator

For assembler, we want the final target to be foo.s (lowercase s).
However, the build.info may have lines like this (note upper case S):

    GENERATE[foo.S]=foo.pl

This indicates that foo.s (lowercase s) is still to be produced, but
that producing it will take an extra step via $(CC) -E.  Therefore,
the following variants (simplified for display) can be generated:

    GENERATE[foo.S]=foo.pl  =>  foo.s: foo.pl
                                	$(PERL) $foo.pl $@.S; \
                                	$(CC) $(CFLAGS) -E -P $@.S > $@ && \
                                	rm -f $@.S

    GENERATE[foo.s]=foo.pl  =>  foo.s: foo.pl
                                	$(PERL) $foo.pl $@

    GENERATE[foo.S]=foo.m4  =>  foo.s: foo.m4
                                	m4 -B 8192 $foo.m4 > $@.S; \
                                	$(CC) $(CFLAGS) -E -P $@.S > $@ && \
                                	rm -f $@.S

    GENERATE[foo.s]=foo.m4  =>  foo.s: foo.m4
                                	m4 -B 8192 $foo.m4 > $@

Reviewed-by: Andy Polyakov <appro@openssl.org>