Reset errno before PTRACE_PEEKUSER for MIPS DSP_CONTROL

PTRACE_PEEKUSER can return -1, which is usually used to determine whether
a system call has reported an error, so errno must be used alone to
determine whether an error occurred. However errno isn't modified by a
successful system call so it must be reset to a known value (0) before the
syscall call.

Add the missing errno reset when reading the DSP_CONTROL register in the
native MIPS Linux backend and the MIPS gdbserver backend.

gdb/:
	* mips-linux-nat.c (mips_linux_read_description): Reset errno to 0
	prior to reading DSP_CONTROL with PTRACE_PEEKUSER ptrace call.

gdb/gdbserver/:
	* linux-mips-low.c (mips_read_description): Reset errno to 0 prior
	to reading DSP_CONTROL with PTRACE_PEEKUSER ptrace call.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2014-09-03  James Hogan  <james.hogan@imgtec.com>
+
+	* mips-linux-nat.c (mips_linux_read_description): Reset errno to 0
+	prior to reading DSP_CONTROL with PTRACE_PEEKUSER ptrace call.
+
 2014-09-03  Sergio Durigan Junior  <sergiodj@redhat.com>
 
 	PR python/16699

gdb/gdbserver/ChangeLog

@@ -1,3 +1,8 @@
+2014-09-03  James Hogan  <james.hogan@imgtec.com>
+
+	* linux-mips-low.c (mips_read_description): Reset errno to 0 prior
+	to reading DSP_CONTROL with PTRACE_PEEKUSER ptrace call.
+
 2014-09-03  Gary Benson  <gbenson@redhat.com>
 
 	* linux-x86-low.c (x86_linux_prepare_to_resume): Use

gdb/gdbserver/linux-mips-low.c

@@ -128,6 +128,7 @@ mips_read_description (void)
     {
       int pid = lwpid_of (current_inferior);
 
+      errno = 0;
       ptrace (PTRACE_PEEKUSER, pid, DSP_CONTROL, 0);
       switch (errno)
 	{

gdb/mips-linux-nat.c

@@ -439,6 +439,7 @@ mips_linux_read_description (struct target_ops *ops)
       if (tid == 0)
 	tid = ptid_get_pid (inferior_ptid);
 
+      errno = 0;
       ptrace (PTRACE_PEEKUSER, tid, DSP_CONTROL, 0);
       switch (errno)
 	{