RISC-V: Add -march=help for gas

Use -march=help for gas to print all supported extensions and versions.

Here is part of the output of `as -march=help`:
All available -march extensions for RISC-V:
        e                                       1.9
        i                                       2.1, 2.0
        m                                       2.0
        a                                       2.1, 2.0
        f                                       2.2, 2.0
        d                                       2.2, 2.0
        q                                       2.2, 2.0
        c                                       2.0
        v                                       1.0
        h                                       1.0
        zicbom                                  1.0
        zicbop                                  1.0
        ...

This patch assumes that the supported extensions with the same versions
are listed together. For example:
static struct riscv_supported_ext riscv_supported_std_ext[] =
{
  ...
  {"i",         ISA_SPEC_CLASS_20191213,        2, 1, 0 },
  {"i",         ISA_SPEC_CLASS_20190608,        2, 1, 0 },
  {"i",         ISA_SPEC_CLASS_2P2,             2, 0, 0 },
  ...
};

For the "i" extension, 2.1.0 with different spec class are listed together.
This patch records the previous printed extension and version.  If the
current extension and version are the same as the previous one, skip
printing.

bfd/
	* elfxx-riscv.c (riscv_print_extensions): New function.  Print
	available extensions and versions.
	* elfxx-riscv.h (riscv_print_extensions): New declaration.
gas/
	* gas/config/tc-riscv.c (md_parse_option): Parse 'help' keyword in
	-march option to print available extensions and versions.
	* testsuite/gas/riscv/march-help.l: New testcase for -march=help.
	* testsuite/gas/riscv/riscv.exp: Updated.