libc/glibc: set the float option according to extra CFLAGS

When building a multilib, some extra CFLAGS can override the
default config option. This is the case for the floating point
selection.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # Compute M68k-specific values

 CT_DoArchTupleValues() {

     :

 }