cmake: Set RX build use 64bit doubles as default

The RX floating-point hardware only works on 32-bit values
so default gcc will build doubles as 32-bit, this make many
test fail on RX build test.
This commit force the build for RX arch to use 64-bit doubles
FPU will be supported later on this arch

Signed-off-by: Duy Nguyen <duy.nguyen.xa@renesas.com>