feat(toolchains): Add riscv64 linux platform to available toolchain platforms

[JeffreyALaw]

This change adds support for the riscv64 linux platform to rules_python. It is one of two changes necessary to allow riscv64 linux platforms to bootstrap modern versions of bazel.

Before this change an attempt to bootstrap bazel would fail with this error:

ERROR: An error occurred during the fetch of repository 'python':
   Traceback (most recent call last):
        File "/home/ubuntu/.cache/bazel/_bazel_ubuntu/c892bce6f2ccb80708b13f24cc8fb67e/external/rules_python/python/private/toolchains_repo.bzl", line 139, column 38, in _toolchain_aliases_impl
                host_platform = get_host_platform(os_name, arch)
        File "/home/ubuntu/.cache/bazel/_bazel_ubuntu/c892bce6f2ccb80708b13f24cc8fb67e/external/rules_python/python/private/toolchains_repo.bzl", line 298, column 13, in get_host_platform
                fail("No platform declared for host OS {} on arch {}".format(os_name, arch))
Error in fail: No platform declared for host OS linux on arch riscv64

This change fixes that error.

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[aignas]

In general looks good to me, but would love this to be mentioned in the CHANGELOG.md file. Could you please update the Added section in there?

[JeffreyALaw]

In general looks good to me, but would love this to be mentioned in the CHANGELOG.md file. Could you please update the Added section in there?

Added. Didn't know if y'all prefer separate commits or a squashed commit, so I left them separate.

[aignas]

The commits get squashed when merged and then the PR description becomes the commit message for the squashed commit.