community/mednafen: update config.sub & add cflags on loongarch64
build error on loongarch64:
>>> ERROR: mednafen*: Found textrels:
TEXTREL /home/pero/aports/community/mednafen/pkg/mednafen/usr/bin/mednafen
>>> ERROR: mednafen*: prepare_subpackages failed
>>> ERROR: mednafen: rootpkg failed
Add the -fPIC compilation option.