linker: introduce on-demand linker section tags

This provides __ondemand_func and __ondemand_rodata tags for annotating
functions and read-only data that should be loaded on demand so not to
exhaust all available RAM. The demand paging mechanism will be leveraged
to load so designated code/data as needed and to evict it from memory
when unused.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>