RISC-V: Handle rouding mode correctly on zfinx

Zfinx has provide fcsr like F, so rouding mode should use fcsr instead
of `soft` fenv.

libgcc/ChangeLog:

	* config/riscv/sfp-machine.h (FP_INIT_ROUNDMODE): Check zfinx.
	(FP_HANDLE_EXCEPTIONS): Ditto.