loader: BIOS loader ld script needs to use KEEP statement with linker sets
The binutils 2.37 has introduced feature to produce "cleaner" binaries, unfortunately it also does affect our BIOS loader build. With current source, the linked ELF binary (loader.sym), has removed linker set functions, because those are not directly referenced. To fix, we need to tell ld to keep linker set symbols, https://wiki.osdev.org/Linker_Scripts#KEEP
Testing done: build with binutils 2.37, confirm the BIOS loader is functional.