riscv: hwprobe: export Zfh[min] ISA extensions

Export Zfh[min] ISA extensions[1] through hwprobe only if FPU support
is available.

Link: https://drive.google.com/file/d/1z3tQQLm5ALsAD77PM0l0CHnapxWCeVzP/view

 [1]
Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Evan Green <evan@rivosinc.com>
Link: https://lore.kernel.org/r/20231114141256.126749-11-cleger@rivosinc.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>