RISC-V: Split riscv_process_target_attr with const char *args argument

RISC-V: Split riscv_process_target_attr with const char *args argument

This patch splits static bool riscv_process_target_attr
(tree args, location_t loc) into two functions:

- bool riscv_process_target_attr (const char *args, location_t loc)
- static bool riscv_process_target_attr (tree args, location_t loc)

Thus, we can call `riscv_process_target_attr` with a `const char *`
argument.  This is useful for implementation of `target_version`
attribute.

gcc/ChangeLog:

	* config/riscv/riscv-protos.h (riscv_process_target_attr): New.
	* config/riscv/riscv-target-attr.cc (riscv_process_target_attr):
	Split into two functions with const char *args argument