altos/stm: Align 'data' to 8 bytes, just like textram

altos/stm: Align 'data' to 8 bytes, just like textram

The textram section must be aligned to 8 bytes to keep the linker
happy. However, if that section contains no data, the declaration will
set the __data_start__ value to that alignment, but the data section
itself would start on a 4-byte alignment, potentially 4 bytes lower
than the value indicated by __data_start__. This completely scrambles
initialized memory as the startup code will copy the data segment to
__data_start__, 4 bytes off of the actual data segment start.

Fix this by forcing the data segment to also be aligned to 8 bytes.

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