loader/efi: Use unique linker set for lua

After the linker set cleanup in ldscripts, there's now only one place we
need to know the linkerset name, so go ahead and change the lua
interpreter augmentation linker set to be uniquely named.

Sponsored by: Netflix
Reviewed by: tsoome, kevans
Differential Revision: https://reviews.freebsd.org/D44062

(cherry picked from commit b4e85f760b6b77d276747c9beb6191bfb864e0c2)