Re-implement ia64-linux-nat.c::ia64_linux_xfer_partial

[description of this patch and ChangeLog entry by Joel Brobecker]
The recent implementation was questionable, and if it worked, it was
only by chance because the requested length is large enough that only
one read was sufficient.  Note that the implementation before that
also made that assumption, in the form of only handling
TARGET_OBJECT_UNWIND_TABLE xfer requests when offset was zero.

gdb/ChangeLog:

        * ia64-linux-nat.c (ia64_linux_xfer_partial): Reimplement
        handling of object == TARGET_OBJECT_UNWIND_TABLE.