Run failed when set GOOS and GOARCH even cross-compile is ignored

[fuxiaohei]

When I try to make support for llgo to build wasm file. It occurs errors but not only in wasm case:

1. In my Mac M1, clone repo, run go build -v github.com/goplus/llgo/cmd/llgo && ./llgo build -o main-binary main.go, everything works fine.
2. If I try GOOS=linux GOARCH=arm64 ./llgo build -o main-binary main.go (use other os and arch pair is same), It shows errors:

c/c_linux.go:25:11: undefined: FilePtr
c/c_linux.go:28:12: undefined: FilePtr
c/c_linux.go:31:12: undefined: FilePtr
...

all llgo/c packages show same type error with undefined type.

3. I found the errors are generated from https://github.com/goplus/llgo/blob/main/internal/packages/load.go#L390

I think we can make it runnable, ignore GOOS and GOARCH to build local target binary; or provide error message to show cross-compile is not supported.