webkit: disable on < ARMv5t

webkit needs the ARM blx instruction, which is only available on >=
ARMv5t, so we have to make sure the webkit package cannot be enabled
on < ARMv5t. In order to achieve this, this commit introduces the
BR2_PACKAGE_WEBKIT_ARCH_SUPPORTS hidden Config.in knob, which avoids
duplicating all over the place the complex architecture dependencies
of webkit.

Fixes:

  http://autobuild.buildroot.org/results/fdf/fdf8bc7660ac251792d0542d2729ea22753d3789/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>