Alien assembly no gc

Half a fix for #1227. The compiler generates a gc-map for `##alien-assembly` but that gc-map is never outputted into the generated code, which you can see if you look at `M: x86 %alien-assembly` in `cpu.x86`. That was the source of the error for the `(sse-version)` case. So what this pr does is just to strip the gc related data from `##alien-assembly`. Because there is no point in outputting gc maps for that instruction ever.