RISC-V: Implement TARGET_CAN_INLINE_P

Currently, we lack support for TARGET_CAN_INLINE_P on the RISC-V
ISA. As a result, certain functions cannot be optimized with inlining
when specific options, such as __attribute__((target("arch=+v"))) .
This can lead to potential performance issues when building
retargetable binaries for RISC-V.

To address this, I have implemented the riscv_can_inline_p function.
This addition enables inlining when the callee either has no special
options or when the some options match, and also ensuring that the
callee's ISA is a subset of the caller's. I also check some other
options when there is no always_inline set.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc (cl_opt_var_ref_t): Add
	cl_opt_var_ref_t pointer to member of cl_target_option.
	(struct riscv_ext_flag_table_t): Add new cl_opt_var_ref_t field.
	(RISCV_EXT_FLAG_ENTRY): New macro to simplify the definition of
	riscv_ext_flag_table.
	(riscv_ext_is_subset): New function to check if the callee's ISA
	is a subset of the caller's.
	(riscv_x_target_flags_isa_mask): New function to get the mask of
	ISA extension in x_target_flags of gcc_options.
	* config/riscv/riscv-subset.h (riscv_ext_is_subset): Declare
	riscv_ext_is_subset function.
	(riscv_x_target_flags_isa_mask): Declare
	riscv_x_target_flags_isa_mask function.
	* config/riscv/riscv.cc (riscv_can_inline_p): New function.
	(TARGET_CAN_INLINE_P): Implement TARGET_CAN_INLINE_P.

Author: cyyself