[arm][doc] Document changes to -mcpu, -mtune and -mfpu.

This patch adds the remainder of the main documentation changes.  It
adds the changes for -mcpu, -mtune and -mfpu.  I've chosen to document
the extension options under -mcpu rather than under -mtune because,
while they are permitted with -mtune, they do not affect the behaviour
of the tuning done by the compiler.

I've also inverted the sense of the table (making the primary index
the extension name and then listing the CPU names to which it applies.
This is because the extensions are much more orthoganal in meaning
here and having a primary entry via the CPU name would lead to
enormous duplication.

Finally, it adds the relevant changes to -mfpu.  I haven't stated yet
that any setting of -mfpu other than 'auto' is deprecated, but that is
certainly the long-term goal of this patch series.

	* doc/invoke.texi (ARM Options, -mcpu): Document supported
	extension options.
	(ARM Options, -mtune): Document that this accepts the same
	extension options as -mcpu.
	(ARM Options, -mfpu): Document addition of -mfpu=auto.

From-SVN: r249308