target: Rewrite read/write buffer default implementations

target: Rewrite read/write buffer default implementations

Rewrite the target_*_buffer_default to generate as large accesses as
possible while maintaining natural alignment.

These versions are easy to extend to generate 8-byte accesses to support
64-bit targets, although it requires some conformity from all target
implementations (i.e. they need to refuse unsupported access sizes with
some defined error code, so we can try again with a smaller one).

Change-Id: I00ddcbb1d2fd33f9f8b99cb448cc93505a2421fc
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1221
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>