Upgrade the C layout of io-page according to MirageOS 3/4

[dinosaure]

According to the plan of MirageOS 4 and the current status of io-page, this PR proposes to upgrade the project to the usual C layout to be compatible with MirageOS 3 and be ready for MirageOS 4.

[dinosaure]

I got an error on alpine:

ld: /home/opam/.opam/4.08/lib/pkgconfig/../../lib/ocaml-freestanding/libasmrun.a(startup_aux_n.o):/home/opam/.opam/4.08/.opam-switch/build/ocaml-freestanding.0.6.4/ocaml/runtime/startup_aux.c:35: multiple definition of `caml_atom_table'; /home/opam/.opam/4.08/lib/pkgconfig/../../lib/ocaml-freestanding/libasmrun.a(startup_nat_n.o):/home/opam/.opam/4.08/.opam-switch/build/ocaml-freestanding.0.6.4/ocaml/runtime/startup_nat.c:47: first defined here
run ['ld' '-nostdlib' '-z' 'max-page-size=0x1000' '-static' '-T'
     '/home/opam/.opam/4.08/lib/pkgconfig/../../lib/solo5-bindings-hvt/solo5_hvt.lds'
     '/home/opam/.opam/4.08/lib/pkgconfig/../../lib/solo5-bindings-hvt/solo5_hvt.o'
     '_build/main.native.o' '_build/manifest.o'
     '-L/home/opam/.opam/4.08/lib/io-page'
     '-l:libiopage_freestanding_stubs.a'
     '/home/opam/.opam/4.08/lib/pkgconfig/../../lib/mirage-solo5/libmirage-solo5_bindings.a'
     '-L/home/opam/.opam/4.08/lib/pkgconfig/../../lib/ocaml-freestanding'
     '-lasmrun' '-lnolibc' '-lopenlibm' '-o' 'io_page.hvt']: exited with 1
run ['dune' 'exec' '--root' '/repo/unikernel' '--' './config.exe' 'build']: exited with 1

But I can not reproduce it on my Ubuntu. Seems like a magic link error 😕 .