zephyr/linker: Move .tbss section back to ROMABLE_REGION

Placing this in RAMABLE_REGION messes up the TLS offsets computed by the
linker, presumably because it's not adjacent to the .tdata section.

In any case, it doesn't really matter as all we want the linker to do is
compute offsets from the TLS base value for all .tdata and .tbss values.

Signed-off-by: Keith Packard <keithp@keithp.com>