toolchain/gcc: Only enable --with-float when it makes sense

According to gcc/config.gcc, only ARM, MIPS and SPARC have the
"--with-float" option when configuring gcc.

[Peter: sort list]
Signed-off-by: Mischa Jonker <mjonker@synopsys.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>