Path for system-wide .eln files (was: Native compilation: the bird-eye view)

[Ulrich Mueller]

Heading over from https://github.com/gentoo/gentoo/pull/16962 to here,
as suggested by Andrea:

System-wide .eln files are currently installed in:
/usr/libexec/emacs/28.0.50/x86_64-pc-linux-gnu/eln-cache/

(I am using /usr here and in the following for better readability,
replace by ${exec_prefix} as appropriate.)

I see two issues with this:

- According to [1] the directory contains ".eln files compiled during
  the build process." Therefore it is not a cache but a normal install
  directory, and "eln-cache" is a misleading name.
  (Or, from a wider perspective, a system-wide cache can never be in
  /usr but should be in /var/cache. And it wouldn't be populated at
  install time but at run time.)

- The FHS [2] says that /usr/libexec is intended for "binaries run
  by other programs". Do .eln files qualify as executable binaries?
  IIUC they are loaded by dlopen, so they're pretty much libraries or
  plugins which should go under /usr/lib{,64} instead.

[1] https://akrl.sdf.org/gccemacs.html#org01229a9
[2] https://refspecs.linuxfoundation.org/FHS_3.0/fhs/ch04s07.html