eco32: switch over to generic elf linker scripts

This moves away from the custom symbol names and makes use of
the names defined in the linker scripts.

Apart from being a cleaner aproach, it also fixes two bugs:
1) having reset exported as global could potentially clash with
user defined symbols called reset.
2) symbols belonging to .bss could end up after the _ebss
symbol, which would prevent them from being cleared by the
bss clearing routine.

Both of those cases occurs in the gcc regression tests.

A special note about the exception vectors,
they are now placed in a special .vectors section
to gain more control over where they are placed.