__builtin_frame_address and __builtin_return_address are unsupported on amd64

[GabrielRavier]

Trying to use __builtin_frame_address or __builtin_return_address simply results in ir/lower/lower_builtins.c:210: libFirm panic in lower_builtin: builtin kind ir_bk_frame_address not supported (for this target) (or ir_bk_return_address for __builtin_return_address)