We need to be able to catch exceptions correctly and thus enable softfloat
on SH4.

As all machines except i386 and x86_64 are using softfloat, make it the
default and change the case to detect i386 and x86_64. Note that CRIS
doesn't have an FPU, so it can be configured with both softfloat-native
and softfloat.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>