tests/functional/test_rx_gdbsim: Use stable URL for test_linux_sash

tests/functional/test_rx_gdbsim: Use stable URL for test_linux_sash

Yoshinori said [*] URL references on OSDN were stable, but they
appear not to be. Mirror the artifacts on GitHub to avoid failures
while testing on CI.

[*] https://www.mail-archive.com/qemu-devel@nongnu.org/msg686487.html

Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Reported-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-ID: <20200630202631.7345-1-f4bug@amsat.org>
[huth: Adapt the patch to the new version in the functional framework]
Message-ID: <20241229083419.180423-1-huth@tuxfamily.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>