i386 loaders: avoid lld 13 garbage collecting linker sets

Because lld 13 and higher default to garbage collecting start/stop
symbols when using --gc-sections, the linker sets used in the i386 boot
loaders will disappear. This leads to the loaders not recognizing any
commands, and failure to boot.

Until we have a good set of linker scripts for the loaders, work around
it by disabling the start-stop-gc feature.

MFC after: 1 week