Linux x86 low-level debug register code synchronization

Linux x86 low-level debug register code synchronization

This commit makes several small changes to the low-level debug
register code for Linux x86, making the code in the GDB and
gdbserver implementations identical.

gdb/ChangeLog:

	* x86-linux-nat.c (x86_linux_dr_set_addr): Update assertion.
	(x86_linux_new_thread): Rename argument.

gdb/gdbserver/ChangeLog:

	* linux-x86-low.c (x86_linux_dr_get): Add assertion.
	Use perror_with_name.  Pass string through gettext.
	(x86_linux_dr_set): Likewise.