make the linker script compatible with lld

To work around a bug in lld, we need to avoid starting the .bss section
from a fixed address (previously, the _sbss symbol). Otherwise lld
incorrectly tries to extend the FLASH output region up to the start of
the RAM output region, which is too large.

To work around another bug in lld, we need to avoid starting sections
marked with (INFO) from a specific starting address. Its parser does not
accept both a start address and the (INFO) attribute.

Specifying these start addresses is redundant anyway, the linker will
lay them out in sequential order for us.