RISC-V: Add new option -march=help to print all supported extensions

The output of -march=help is like below:

```
All available -march extensions for RISC-V:
        Name                Version
        i                       2.0, 2.1
        e                       2.0
        m                       2.0
        a                       2.0, 2.1
        f                       2.0, 2.2
        d                       2.0, 2.2
...
```

Also support -print-supported-extensions and --print-supported-extensions for
clang compatibility.

gcc/ChangeLog:

	PR target/109349

	* common/config/riscv/riscv-common.cc (riscv_arch_help): New.
	* config/riscv/riscv-protos.h (RISCV_MAJOR_VERSION_BASE): New.
	(RISCV_MINOR_VERSION_BASE): Ditto.
	(RISCV_REVISION_VERSION_BASE): Ditto.
	* config/riscv/riscv-c.cc (riscv_ext_version_value): Use enum
	rather than magic number.
	* config/riscv/riscv.h (riscv_arch_help): New.
	(EXTRA_SPEC_FUNCTIONS): Add riscv_arch_help.
	(DRIVER_SELF_SPECS): Handle -march=help, -print-supported-extensions and
	--print-supported-extensions.
	* config/riscv/riscv.opt (march=help): New.
	(print-supported-extensions): New.
	(-print-supported-extensions): New.
	* doc/invoke.texi (RISC-V Options): Document -march=help.

Reviewed-by: Christoph Müllner <christoph.muellner@vrull.eu>