[docs] Use x86_64 and i386 instead of x86 as arch for triples.

[fhahn]

Summary

x86 is not a valid arch for target triples, but x86_64 and i386 are.

[fhahn]

Ping. This is a two-line documentation fix, but I want to make sure x86_64 and i386 are valid for arch in target triples in all cases.

[silvas]

LGTM. Do you need me to commit this for you?

[fhahn]

Thanks, I can commit it myself, I just wanted to be extra careful when changing something x86 related.

cfe/trunk/docs/CrossCompilation.rst

 
 The basic option is to define the target architecture. For that, use
 ``-target <triple>``. If you don't specify the target, CPU names won't
 match (since Clang assumes the host triple), and the compilation will
 go ahead, creating code for the host platform, which will break later
 on when assembling or linking.
 
 The triple has the general format ``<arch><sub>-<vendor>-<sys>-<abi>``, where:
- * ``arch`` = ``x86``, ``arm``, ``thumb``, ``mips``, etc.
+ * ``arch`` = ``x86_64``, ``i386``, ``arm``, ``thumb``, ``mips``, etc.
  * ``sub`` = for ex. on ARM: ``v5``, ``v6m``, ``v7a``, ``v7m``, etc.
  * ``vendor`` = ``pc``, ``apple``, ``nvidia``, ``ibm``, etc.
  * ``sys`` = ``none``, ``linux``, ``win32``, ``darwin``, ``cuda``, etc.
  * ``abi`` = ``eabi``, ``gnu``, ``android``, ``macho``, ``elf``, etc.
 
 The sub-architecture options are available for their own architectures,
-of course, so "x86v7a" doesn't make sense. The vendor needs to be 
+of course, so "x86v7a" doesn't make sense. The vendor needs to be
 specified only if there's a relevant change, for instance between PC
 and Apple. Most of the time it can be omitted (and Unknown)
 will be assumed, which sets the defaults for the specified architecture.
 The system name is generally the OS (linux, darwin), but could be special
 like the bare-metal "none".
 
 When a parameter is not important, it can be omitted, or you can
 choose ``unknown`` and the defaults will be used. If you choose a parameter