[bug#41167] Julia: fix current patches, upgrade to 1.4.1

[Nicolò Balzarotti]

Hello guix!

In the following patches I:
- fixed current julia: I don't know how, the patch-url is missing
  ".patch" at the end, leading to a 404.  Don't know how it can build
  right now
- fixed the julia-SOURCE_DATE_EPOCH-mtime.patch: It's now a meta-patch,
  that creates the right patch file, so the current julia version is not
  patched, and precompile cache of packages installed with guix is
  broken.  Still no julia-package is in guix, that's why it went
  unnoticed
- Add libwhich (I added it to julia.scm but probably there's a better
  place for it)
- Update julia to latest release, v1.4.1.  I was finally able to
  unbundle the last pieces of bundled software (libwhich).  Now julia
  is based on llvm-8 (previously on llvm-6) and I measuread a speedup of
  ~17x on precompile times
- Add the first julia package, julia-compat.  If everything is fine, I'll
  then submit a big series of packages

Feedback welcome!

Nicolò