gdb/linux_nat: Write memory using ptrace if /proc/pid/mem is not writable

Commit 05c06f31 enabled GDB to access
memory while threads are running.  It did this by accessing
/proc/PID/task/LWP/mem.

Unfortunately, this interface is not implemented for writing in older
kernels (such as RHEL6).  This means that GDB is unable to insert
breakpoints on these hosts:

 $ ./gdb -q gdb -ex start
 Reading symbols from gdb...
 Temporary breakpoint 1 at 0x40fdd5: file ../../src/gdb/gdb.c, line 28.
 Starting program: /home/rhel6/fsf/linux/gdb/gdb
 Warning:
 Cannot insert breakpoint 1.
 Cannot access memory at address 0x40fdd5

 (gdb)

Before this patch, linux_proc_xfer_memory_partial (previously called
linux_proc_xfer_partial) would return TARGET_XFER_EOF if the write to
/proc/PID/mem failed. [More specifically, linux_proc_xfer_partial
would not "bother for one word," but the effect is the essentially
same.]

This status was checked by linux_nat_target::xfer_partial, which would
then fallback to using ptrace to perform the operation.

This is the specific hunk that removed the fallback:

-  xfer = linux_proc_xfer_partial (object, annex, readbuf, writebuf,
-                                 offset, len, xfered_len);
-  if (xfer != TARGET_XFER_EOF)
-    return xfer;
+      return linux_proc_xfer_memory_partial (readbuf, writebuf,
+                                            offset, len, xfered_len);
+    }

   return inf_ptrace_target::xfer_partial (object, annex, readbuf, writebuf,
                                          offset, len, xfered_len);

This patch makes linux_nat_target::xfer_partial go straight to writing
memory via ptrace if writing via /proc/pid/mem is not possible in the
running kernel, enabling GDB to insert breakpoints on these older
kernels.  Note that a recent patch changed the return status from
TARGET_XFER_EOF to TARGET_XFER_E_IO.

Tested on {unix,native-gdbserver,native-extended-gdbserver}/-m{32,64}
on x86_64, s390x, aarch64, and ppc64le.

Change-Id: If1d884278e8c4ea71d8836bedd56e6a6c242a415