mips: Fix store/load gp registers to/from ucontext_t

General purpose registers in mcontext_t structure
are 8 bytes long for both MIPS32/MIPS64.

get/set/make/swap context implementations for MIPS O32
incorrectly assume that general purpose registers
in this structure are 4 bytes long.

This patch is fixing that.

Tested for MIPS O32 LE and BE.
Compared objdump of modified functions for mips n32 and mips n64.

	[BZ #21548]
	* sysdeps/unix/sysv/linux/mips/getcontext.S: Define MCONTEXT_SZGREG as
	8 and use it when copying general purpose registers.
	* sysdeps/unix/sysv/linux/mips/makecontext.S: Likewise.
	* sysdeps/unix/sysv/linux/mips/mips32/Makefile: Include new test for
	mips o32.
	* sysdeps/unix/sysv/linux/mips/mips32/bug-getcontext-mips-gp.c: Added
	new test for mips o32.
	* sysdeps/unix/sysv/linux/mips/setcontext.S: Define MCONTEXT_SZGREG as
	8 and use it when copying general purpose registers.
	* sysdeps/unix/sysv/linux/mips/swapcontext.S: Likewise.

sysdeps/unix/sysv/linux/mips/mips32/bug-getcontext-mips-gp.c

@@ -0,0 +1,63 @@
+/* Tests register values retreived by getcontext() for mips o32.
+   Copyright (C) 2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <ucontext.h>
+
+
+#if !defined __mips__ || _MIPS_SIM != _ABIO32
+# error "MIPS O32 specific test."
+#endif
+
+#define SP_REG 29
+
+static int
+do_test (void)
+{
+  ucontext_t ctx;
+  memset (&ctx, 0, sizeof (ctx));
+  int status = getcontext (&ctx);
+  if (status)
+    {
+      printf ("\ngetcontext() failed, errno: %d.\n", errno);
+      return 1;
+    }
+
+  if (ctx.uc_mcontext.gregs[SP_REG] == 0
+      || ctx.uc_mcontext.gregs[SP_REG] > 0xffffffff)
+    {
+      printf ("\nError getcontext(): invalid $sp = 0x%llx.\n",
+              ctx.uc_mcontext.gregs[SP_REG]);
+      return 1;
+    }
+
+  if (ctx.uc_mcontext.pc == 0
+      || ctx.uc_mcontext.pc > 0xffffffff)
+    {
+      printf ("\nError getcontext(): invalid ctx.uc_mcontext.pc = 0x%llx.\n",
+              ctx.uc_mcontext.pc);
+      return 1;
+    }
+
+  return 0;
+}
+
+#include <support/test-driver.c>