config/target: add float support selection

Changeset #149c33923f47 broke the architectures that do not
support the --with-float=X ./configure flag (in gcc). For example,
x86_64 does not support it.

Add a new blind config option that architectures can set to tell
they support floating point selection.

Reported-by: Morten Thunberg Svendsen <mts@doredevelopment.dk>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

# m68k specific configuration file

## depends on EXPERIMENTAL

##

## select ARCH_SUPPORTS_32

## select ARCH_DEFAULT_32

## select ARCH_DEFAULT_BE

## select ARCH_SUPPORTS_WITH_CPU

##

## help The m68k architecture