RISC-V: Refactor riscv_option_override and riscv_convert_vector_bits. [NFC]

Allow those funciton apply from a local gcc_options rather than the
global options.

Preparatory for target attribute, sperate this change for eaiser reivew
since it's a NFC.

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_convert_vector_bits): Get setting
	from argument rather than get setting from global setting.
	(riscv_override_options_internal): New, splited from
	riscv_override_options, also take a gcc_options argument.
	(riscv_option_override): Splited most part to
	riscv_override_options_internal.