fbdump: only build VGA16 support on x86/x86-64

The VGA16 dumping feature probably doesn't make much sense on anything
else than x86 and x86-64, as it is accessing I/O ports that seem
PC-specific.

Moreover, some architectures such as SH4, do not have an
implementation of outw() in uClibc, but we quite certainly don't care
about VGA16 support for such architectures.

Fixes:

  http://autobuild.buildroot.org/results/e1c860830dd585830cf5ca5f935b81856e992334/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>