ctf: handle structure and union offsets in form DW_FORM_data1

ctf: handle structure and union offsets in form DW_FORM_data1

GCC 4.8.0 and above have started to represent structure member offsets in the
DW_AT_data_member_location attribute using DWARF forms DW_FORM_data1 and _data2
where possible: indeed, this is the common case, as most structure members are
less than 64K into the structure: and every structure contains at least one
member whose offset is represented this way.

dwarf2ctf neglected to check for DW_FORM_data1, so the build failed when it hit
the first structure in the kernel and realised that it didn't understand its
offset.

Fix trivial.

Orabug: 18117464
Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Reviewed-by: Chuck Anderson <chuck.anderson@oracle.com>