x11-wm/gamescope: unexpand powerpc* after 6a56ecb3558d

Remove endian suffix instead of hardcoding ARCH abbreviations. Meson
doesn't infer endianess from ARCH while FreeBSD removed big-endian
except for PowerPC and inconsitently used "el" and "le" as suffix.
For example, gamescope requires modern compiler and KMS-based GPU
drivers, so "armeb", "mips64el", "sparc64" were never supported.