_vector_table and __vector_relay_table symbols were exported with GTEXT
(i.e. as functions). That resulted in bit[0] being incorrectly set in
the addresses they represent (for functions this bit set to 1 specifies
execution in Thumb state).
This commit corrects this by switching to exporting these objects as
objects, i.e. with GDATA.
Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>