PULP_FC_ARCH_CFLAGS is not used by pulp-runtime

`-march` cflag is explicitly specified in

https://github.com/pulp-platform/regression_tests/blob/master/riscv_tests/testComplex/Makefile

```Makefile
PULP_FC_ARCH_CFLAGS = -march=rv32imcxgap9 -D__riscv__
```

But `PULP_FC_ARCH_CFLAGS` is not used by pulp-runtime, a similar Makefile var name is `PULP_ARCH_CFLAGS`