[NEW PORT] games/libretro-mu: PalmOS emulator to the libretro API

Mu is a PalmOS emulator core for libretro compatible emulators, such as games/retroarch.

PR: 246148
Submitted by: menelkir@itroll.org
Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D24863