Update manual/contrib.texi.

Thank Yinyu Cai for their maintainership of the LoongArch port.

Thank Vineet Gupta for their maintainership of the ARC port.

Thank Tulio Magno Quites Machado Filho for their past maintainership
of the PowerPC port.

Thank Rajalakshmi Srinivasaraghavan for their current maintainership
of the PowerPC port.