Update PKGBUILD

*Update libgcj12 revision*

Hi, I changed the PKGBUILD of libgcj12, as the current one returns a 404 NOT FOUND error while downloading the Debian package.
I bumped the package revision to 4.6.3_4 and changed MD5sums, as the libgcj12_4.6.3_3 is not on the Debian mirrors anymore (it returns a 404 error).