[fix, build] AppImage use `-mtune=generic -march=x86-64` (#644)
[fix, build] AppImage use `-mtune=generic -march=x86-64` (#644)

`-march=native` is good for the emulator but not for an AppImage that needs to support various CPU models.

References #3853.
NiLuJe/koreader-base,koreader/koreader-base,Frenzie/koreader-base,NiLuJe/koreader-base,NiLuJe/koreader-base,Frenzie/koreader-base,houqp/koreader-base,houqp/koreader-base,koreader/koreader-base,NiLuJe/koreader-base,koreader/koreader-base,koreader/koreader-base,houqp/koreader-base,houqp/koreader-base,Frenzie/koreader-base,Frenzie/koreader-base