Before this patch:

    $ gpython
    Python 3.13.5 (main, Jun 25 2025, 18:55:22) [GCC 14.2.0] [GPython 0.1] [gevent 25.9.1] on linux
    Type "help", "copyright", "credits" or "license" for more information.
    >>>

After this patch:

    $ gpython
    Python 3.13.5 (main, Jun 25 2025, 18:55:22) [GCC 14.2.0] [GPython 0.1] [gevent 25.9.1] on linux/riscv64
    Type "help", "copyright", "credits" or "license" for more information.
    >>>

Note that we try not to use golang.runtime when ran not under gpython
via plain pymain. There we use platform.machine() instead of
runtime.ARCH, but the output can be different - e.g. aarch64 instead of
arm64. In general when running under gpython we follow go convention for
both OS and ARCH, e.g. android, not linux for an Android phone. But when
running under plain python and pymain we stick to sys.platform and
platform.machine() to avoid importing golang on workloads that do not
use it. Plain pymain output for above example looks as:

    $ pymain
    Python 3.13.5 (main, Jun 25 2025, 18:55:22) [GCC 14.2.0] on linux/riscv64
    Type "help", "copyright", "credits" or "license" for more information.
    >>>

/reviewed-by @jerome
/reviewed-on !38