ARM: remove sub-arch/variants selection from menuconfig

ARM: remove sub-arch/variants selection from menuconfig

Rely on the compiler to be correctly set up to generate
appropriate code for the target variant.

This exposes the Thumb option, as it is no longer auto-selected.
The "Use BX" no longer depends on supported CPU to be selected,
so it now defaults to 'n' as it shall work by default on CPUs
that do not have BX.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Cc: Khem Raj <raj.khem@gmail.com>
Cc: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>
Cc: Carmelo AMOROSO <carmelo.amoroso@st.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>