SW Build fails with vexriscv minimal variant

Noticed this while doing some other experimenting but the following error occurs when running `make software EXTRA_LITEX_ARGS="--cpu-variant=minimal"` out of the box but works for `minimal+cfu` variant:

```
/..../CFU-Playground/env/conda/envs/cfu-symbiflow/bin/../lib/gcc/riscv32-elf/10.1.0/../../../../riscv32-elf/bin/ld: /..../CFU-Playground/proj/proj_template/build/src/tensorflow/lite/micro/micro_utils.cc:89: undefined reference to `__truncdfsf2'
/..../CFU-Playground/env/conda/envs/cfu-symbiflow/bin/../lib/gcc/riscv32-elf/10.1.0/../../../../riscv32-elf/bin/ld: /..../CFU-Playground/proj/proj_template/build/src/tensorflow/lite/micro/micro_utils.cc:78: undefined reference to `__truncdfsf2'
/..../CFU-Playground/env/conda/envs/cfu-symbiflow/bin/../lib/gcc/riscv32-elf/10.1.0/../../../../riscv32-elf/bin/ld: src/tensorflow/lite/micro/test_helpers.o: in function `void tflite::SymmetricPerChannelQuantize(float const*, long*, int, int, float*)':
/..../CFU-Playground/proj/proj_template/build/src/tensorflow/lite/micro/micro_utils.h:96: undefined reference to `__extendsfdf2'
/..../CFU-Playground/env/conda/envs/cfu-symbiflow/bin/../lib/gcc/riscv32-elf/10.1.0/../../../../riscv32-elf/bin/ld: src/tensorflow/lite/micro/test_helpers.o: in function `long long tflite::FloatToSymmetricQuantizedType(float, float)':
/..../CFU-Playground/proj/proj_template/build/src/tensorflow/lite/micro/micro_utils.h:55: undefined reference to `__extendsfdf2'
collect2: error: ld returned 1 exit status
make[3]: *** [Makefile:166: software.elf] Error 1
make[3]: Leaving directory '/..../CFU-Playground/proj/proj_template/build'
make[2]: *** [../proj.mk:233: /..../CFU-Playground/proj/proj_template/build/software.bin] Error 2
make[2]: Leaving directory '/..../CFU-Playground/proj/proj_template

```