toolchain: select sensible floating point mode by default

Introduce BR2_PREFER_SOFT_FLOAT and enable it for arm, avr32 and mips,
and use it to enable soft float support by default for those archs.

This way people automatically get the correct floating point mode when
running make menuconfig for the first time (E.G. without a .config) and
changing the arch from i386 to arm/avr32/mips.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>