x86 Linux/ptrace: fix offsetof usage in C++ mode

x86 Linux/ptrace: fix offsetof usage in C++ mode

In C++ mode, we get:

  gdb/gdbserver/linux-x86-low.c: In function ‘void x86_linux_dr_set(ptid_t, int, long unsigned int)’:
  gdb/gdbserver/linux-x86-low.c:558:38: error: ‘regnum’ cannot appear in a constant-expression
      offsetof (struct user, u_debugreg[regnum]), value);
                                      ^
gdb/gdbserver/ChangeLog:
2015-02-27  Pedro Alves  <palves@redhat.com>

	* linux-x86-low.c (u_debugreg_offset): New function.
	(x86_linux_dr_get, x86_linux_dr_set): Use it.

gdb/ChangeLog:
2015-02-27  Pedro Alves  <palves@redhat.com>

	* x86-linux-nat.c (u_debugreg_offset): New function.
	(x86_linux_dr_get, x86_linux_dr_set): Use it.