package/gauche: bump to version 0.9.12

- Update site to get latest release
- Update hash of COPYING (update in year with
  https://github.com/shirok/Gauche/commit/09a3d4f9da52d8b3d9bc28a0ff2aa9c02e99190a
  https://github.com/shirok/Gauche/commit/2a4146798cdaceaf43764e1bf7b959b4a31bf995)

https://github.com/shirok/Gauche/blob/release0_9_12/ChangeLog
https://practical-scheme.net/gauche/gmemo/index.cgi?Release%200.9.12



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>