RISC-V: Implement new style of architecture extension test macros.

RISC-V: Implement new style of architecture extension test macros.

- This patch introduce new set of architecture extension test macros
  which is accept on riscv-c-api-doc recently.
  - https://github.com/riscv/riscv-c-api-doc/blob/master/riscv-c-api.md#architecture-extension-test-macro

- We will also mark deprecated for legacy architecture extension test macros
  in GCC 11, but still support that for 1 or 2 release cycles.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.c (riscv_current_subset_list): New.
	* config/riscv/riscv-c.c (riscv-subset.h): New.
	(INCLUDE_STRING): Define.
	(riscv_cpu_cpp_builtins): Add new style architecture extension
	test macros.
	* config/riscv/riscv-subset.h (riscv_subset_list::begin): New.
	(riscv_subset_list::end): New.
	(riscv_current_subset_list): New.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/predef-10.c: New.
	* gcc.target/riscv/predef-11.c: New.
	* gcc.target/riscv/predef-12.c: New.
	* gcc.target/riscv/predef-13.c: New.