Rename 32- and 64-bit Intel files from "i386" to "x86"

This commit renames six files that contain code used by both 32- and
64-bit Intel ports such that their names are prefixed with "x86"
rather than "i386".  All types, functions and variables within these
files are likewise renamed such that their names are prefixed with
"x86" rather than "i386".  This makes GDB follow the convention used
by gdbserver such that 32-bit Intel code lives in files called
"i386-*", 64-bit Intel code lives in files called "amd64-*", and code
for both 32- and 64-bit Intel lives in files called "x86-*".

This commit only renames OS-independant files.  The Linux ports of
both GDB and gdbserver now follow the i386/amd64/x86 convention fully.
Some ports still use the old convention where "i386" in file/function/
type/variable names can mean "32-bit only" or "32- and 64-bit" but I
don't want to touch ports I can't fully test except where absolutely
necessary.

gdb/ChangeLog:

	* i386-nat.h: Renamed as...
	* x86-nat.h: New file.  All type, function and variable name
	prefixes changed from "i386_" to "x86_".  All references updated.
	* i386-nat.c: Renamed as...
	* x86-nat.c: New file.  All type, function and variable name
	prefixes changed from "i386_" to "x86_".  All references updated.
	* nat/i386-dregs.h: Renamed as...
	* nat/x86-dregs.h: New file.  All type, function and variable name
	prefixes changed from "i386_" to "x86_".  All references updated.
	* nat/i386-dregs.c: Renamed as...
	* nat/x86-dregs.c: New file.  All type, function and variable name
	prefixes changed from "i386_" to "x86_".  All references updated.

gdb/gdbserver/ChangeLog:

	* i386-low.h: Renamed as...
	* x86-low.h: New file.  All type, function and variable name
	prefixes changed from "i386_" to "x86_".  All references updated.
	* i386-low.c: Renamed as...
	* x86-low.c: New file.  All type, function and variable name
	prefixes changed from "i386_" to "x86_".  All references updated.