gdb: rename target_read_auxv(target_ops *) to target_read_auxv_raw

Having two overloads of target_read_auxv that don't have the same goals
is confusing.  Rename the one that reads from an explicit target_ops to
target_read_auxv_raw.  Also, it occured to me that the non-raw version
could use the raw version, that reduces duplication a bit.

Change-Id: I28e5f7cecbfcacd0174d4686efb3e4a23b4ad491