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>

# powerpc specific configuration file

## select ARCH_SUPPORTS_32

## select ARCH_SUPPORTS_64 if EXPERIMENTAL

## select ARCH_DEFAULT_32

## select ARCH_USE_MMU

## select ARCH_SUPPORTS_WITH_ABI

## select ARCH_SUPPORTS_WITH_CPU

## select ARCH_SUPPORTS_WITH_TUNE

##

## help The PowerPC architecture, as defined by:

## help     http://www.ibm.com/developerworks/eserver/articles/archguide.html