riscv: support building double-float modules

The riscv32 toolchain for GCC-12 provided by kernel.org contains libgcc.a
compiled for double-float. To link to it we have to adjust how we build
U-Boot.

As U-Boot actually does not use floating point at all this should not
make a significant difference for the produced binaries.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>