Clarify the %{system} values for Linux

Author: dra27

lib_eio_linux/dune

@@ -2,10 +2,10 @@
  (name eio_linux)
  (public_name eio_linux)
  (enabled_if ; See https://github.com/ocaml/dune/issues/4895
-   (or (= %{system} "linux")          ; Linux-x86
-       (= %{system} "linux_eabihf")   ; Linux-arm32
-       (= %{system} "linux_elf")      ; Linux-x86_32
-       (= %{system} "elf")))          ; Linux-ppc64
+   (or (= %{system} "linux")          ; Historically, just Linux-x86
+       (= %{system} "linux_eabihf")   ; Historically, Linux-arm32
+       (= %{system} "linux_elf")      ; Historically, Linux-x86_32
+       (= %{system} "elf")))          ; Historically, Linux-ppc64
  (foreign_stubs
   (language c)
   (flags :standard -D_LARGEFILE64_SOURCE)