arc: linker.ld: fix BSS section declaration

The BSS section needs to use AT> in XIP systems otherwise the LMA
addresses in the ELF binary are wrong, leading to issues if we
try to manipulate the binary with objcopy. The GROUP_DATA_LINK_IN
macro does the right thing here.

This was already done on other arches but ARC was missed.

Change-Id: I93748e919e0b68c1ff2dfb4b85b7064a8d980f3a
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>