[PATCH v3] RISC-V: Implement RISC-V profile macro supportHEADtrunkmaster

users can now write code like the following to adapt to the
current RISC-V profile selected at compile time:

```c
  #ifdef __riscv_rva23u64
    // Code specific to the rva23u64 profile
  #endif
```

Changes from v2:
	- clarify get_profile_name comment

gcc/
	* common/config/riscv/riscv-common.cc (riscv_subset_list::get_profile_name):
	New function.
	* config/riscv/riscv-c.cc (riscv_cpu_cpp_builtins): Define
	profile macro if a profile is detected.
	* config/riscv/riscv-subset.h (riscv_subset_list::get_profile_name): Declare.

gcc/testsuite/
	* gcc.target/riscv/predef-profiles-1.c: New test for __riscv_rvi20u64.
	* gcc.target/riscv/predef-profiles-2.c: New test for __riscv_rvi20u32.
	* gcc.target/riscv/predef-profiles-3.c: New test for __riscv_rva20u64.
	* gcc.target/riscv/predef-profiles-4.c: New test for __riscv_rva22u64.
	* gcc.target/riscv/predef-profiles-5.c: New test for __riscv_rva23u64.
	* gcc.target/riscv/predef-profiles-6.c: New test for __riscv_rva23s64.
	* gcc.target/riscv/predef-profiles-7.c: New test for __riscv_rvb23u64.
	* gcc.target/riscv/predef-profiles-8.c: New test for __riscv_rvb23s64.

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