Do garbage collection of unused sections.

Do garbage collection of unused sections.

Implement -ffunction-sections and -fdata-sections in both 32bit and
    16bit code.
Make sure all sections have unique names (even asm and discarded
    sections).
Enhance tools/layoutrom.py script to find all sections reachable from
    exported 16bit code - prune all other sections.
Mark sections with "export" if they can be visible outside of code -
    these sections wont be dropped when pruning unused sections.