arch: introduce BR2_BINFMT_SUPPORTS_SHARED

In preparation for the refactoring of the static/shared library
support, we add a BR2_BINFMT_SUPPORTS_SHARED hidden option that binary
formats supporting shared libraries should select.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>