Fix architecture detection when using 32bit mode on ARM64 and i486

[kit-ty-kate]

armv8l and armv8b actually mean "32bit emulation on an armv8 processor", see https://reviews.llvm.org/rG9b958356983afffaf56788f37bdab9213369fa45

This was causing issue in ocaml/opam-repository#17747 where the package does not work on Arm32, restricted the availability there but 32bit environment is still wrongly detected as arm64:

#=== ERROR while compiling albatross.1.0.1 ====================================#
# context              2.0.7 | linux/arm64 | ocaml-base-compiler.4.11.1 | pinned(https://github.com/roburio/albatross/releases/download/v1.0.1/albatross-v1.0.1.tbz)
# path                 ~/.opam/4.11/.opam-switch/build/albatross.1.0.1
# command              ~/.opam/4.11/bin/dune build -p albatross -j 31
# exit-code            1
# env-file             ~/.opam/log/albatross-7-6a4d96.env
# output-file          ~/.opam/log/albatross-7-6a4d96.out
### output ###
#     discover stats/c_flags.sexp,stats/c_library_flags.sexp (exit 2)
# (cd _build/default/stats && config/discover.exe)
# Fatal error: exception Failure("Unsupported platform: linux_eabihf")

Test-case:

$ docker run --rm -it --entrypoint bash ocaml/opam:debian-10-ocaml-4.11@sha256:ca5b021f7b74707f3d5df69657bb5a34062e81f0cfb71fdf8101cb4b32a8980a
opam@3440e642156a:~$ uname -m
aarch64
opam@3440e642156a:~$ linux32 uname -m
armv8l

[rjbou]

Thanks!