Fix linking of backup RAM and bss segments

After the linkscript update to support large P-ROMs [1,2],
the ordering of the bss and backup RAM segments has changed,
and this broke the logic used to compute the start of the .bss
segment in RAM. Fix accordingly.

Closes dciabrin#82

[1] a83e751
[2] db99afd

Fix init of .bss and .data in crt0

After dciabrin#78 and dciabrin#81, the .bss segment is now linked prior to the
.data segment. The crt0 still assumes the opposite, so the
memory is incorrectly initialized.

Since the linker now exposes symbol __bss_start_in_ram, the crt0
no longer needs to assume any ordering. Fix it accordingly.

Initial release tag