libunwind: Fix build on arm < v6

atomics that are needed for libunuwind are not available on
arm < v6

Fixes errors like

| ../src/.libs/libunwind.so: error: undefined reference to
'AO_fetch_and_add1'
| ../src/.libs/libunwind.so: error: undefined reference to
'AO_fetch_and_add'
| collect2: error: ld returned 1 exit status

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>