loader/efi: Only include interpreter's linker script

For safety, only include the interpreter's linker script. Note that the
simple loader doesn't have one, but it's not an error to copy a ELF
section that does not exist. No functional change, however.

Sponsored by: Netflix
Reviewed by: tsoome, kevans
Differential Revision: https://reviews.freebsd.org/D44064

(cherry picked from commit f5f08e41aa57f94e3c127a488a82b4e518781f89)