check-uapi: link into shared objects

While testing ABI changes across all architectures, I found that abidiff
sometimes produces nonsensical output. Further debugging identified
missing or broken libelf support for architecture specific relocations
in ET_REL binaries as the source of the problem[1].

Change the script to no longer produce a relocatable object file but
instead create a shared library for each header. This makes abidiff
work for all of the architectures in upstream linux kernels.

Link: https://sourceware.org/bugzilla/show_bug.cgi?id=33869
Cc: stable@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20260306163309.2015837-2-arnd@kernel.org
Signed-off-by: Nicolas Schier <nsc@kernel.org>