loader/ficl: Rename the ficl compile set to X4th_compile_set

And upcoming change will need this set to be named this. Since it's only
used in the efi Makefile, and inside if ficl itself, the change should
be a nop.

Sponsored by: Netflix
Reviewed by: tsoome, kevans
Differential Revision: https://reviews.freebsd.org/D44063