boost: remove BR2_PACKAGE_BOOST_ARCH_SUPPORTS option

boost: remove BR2_PACKAGE_BOOST_ARCH_SUPPORTS option

The BR2_PACKAGE_BOOST_ARCH_SUPPORTS option was originally added in
commit feeab03fa68a02733ae9382b7d47d9eb0d785188 to be able to disable
Boost on broken NIOSII CodeSourcery toolchains.

However, since then, the CodeSourcery toolchain has been updated, and
once the fenv problem is fixed, this NIOSII toolchain is capable of
building Boost.

Thanks to this we can completely get rid of the
BR2_PACKAGE_BOOST_ARCH_SUPPORTS symbol, from boost itself and from all
its reverse dependencies.

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