Log Message:
emulators/libretro-picodrive: Update to 20180722.

According to picodrive's author, the code is unsuitable for big endian
platforms. Libretro doesn't provide Picodrive binaries for any of their
supported big endian platforms (mostly PowerPC games consoles). Instead,
they provide Genesis Plus GX, which I shall add to pkgsrc soon(TM).

For now, disable building libretro-picodrive on big endian.

No upstream changelog, unfortunately.