kbuild: refactor variables in scripts/link-vmlinux.sh

Clean up the variables in scripts/link-vmlinux.sh

 - Specify the extra objects directly in vmlinux_link()
 - Move the AS rule to kallsyms()
 - Set kallsymso and btf_vmlinux_bin_o where they are generated
 - Remove unneeded variable, kallsymso_prev
 - Introduce the btf_data variable
 - Introduce the strip_debug flag instead of checking the output name

No functional change intended.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>