Improve how dynamic_big_crypt.c is generated

The source file dynamic_big_crypt.c is made at make time, from many parts and with substitutions for build target (such as SIMD type and so on).

Makefile.in

dynamic_big_crypt.c: dynamic_big_crypt_hash.cin dynamic_big_crypt_header.cin dynamic_big_crypt_generator.sh dynamic_big_crypt_chopper.pl unused/dynamic_big_crypt.c
        $(shell ./dynamic_big_crypt_generator.sh)
        @if [ ! -f dynamic_big_crypt.c ] ; then $(CP) unused/dynamic_big_crypt.c dynamic_big_crypt.c ; fi

There seems to be a fallback in the above, copying a file from unused/ (which BTW is a poor location to keep a fallback) but this apparently backfired somehow in #4856 (where the problem was lack of perl).

src/dynamic_big_crypt_generator.sh in turn runs src/dynamic_big_crypt_chopper.pl introducing the need for perl. The perl script was introduced in 0aace8a0c, replacing dynamic_big_crypt_chopper.c which was apparently a C program doing the job - which had some problems with cross compiles according to this comment:

# Ported to Perl at request of Magnum. We were seeing build/run issues
# on cross complier environments.

See also #4868

Start with the dynamic_big_crypt.c rule in Makefile.in, then read dynamic_big_crypt_generator.sh, dynamic_big_crypt_chopper.pl, and the related .cin files. Trace how substitutions and the unused/dynamic_big_crypt.c fallback behave, especially when Perl is unavailable or builds are cross-compiled. Done should mean the generation path is reliable for the supported build targets without the reported fallback problem.