RISC-V: warning: fedisableexcept is not implemented and will always fail

Cross-compiling a minimal Boost Test example for RISC-V:
```cpp
#define BOOST_TEST_MODULE Example Test
#include

BOOST_AUTO_TEST_CASE( test1 )
{
int i = 1;
BOOST_CHECK( i*i == 1 );
}
```

```bash
riscv64-linux-gnu-g++-11 --version
riscv64-linux-gnu-g++-11 (Ubuntu 11.2.0-16ubuntu1) 11.2.0

riscv64-linux-gnu-g++-11 example.cpp
/usr/lib/gcc-cross/riscv64-linux-gnu/11/../../../../riscv64-linux-gnu/bin/ld: /tmp/ccvQglzN.o: in function `.L0 ':
riscv_boost_test.cpp:(.text+0xd300): warning: fedisableexcept is not implemented and will always fail
/usr/lib/gcc-cross/riscv64-linux-gnu/11/../../../../riscv64-linux-gnu/bin/ld: riscv_boost_test.cpp:(.text+0xd2ac): warning: feenableexcept is not implemented and will always fail
```