ARM: make configuration of userspace Thumb support an expert option

David Mosberger reports random segfaults and other problems when running
his buildroot userspace.  It turns out that his kernel did not have
support for Thumb userspace, nor did his application, but glibc made use
of Thumb instructions in glibc.

The kernel Thumb support option already recommends being enabled, and
is also so biased, but clearly this is not enough of a recommendation.

So, hide this behind CONFIG_EXPERT as well, and include a note to
indicate the potential issues if it's turned off and userspace Thumb
mode is made use of.

Reported-by: David Mosberger <davidm@egauge.net>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>