add wasm-abi field in TargetSpec && set generic for WASI by default (#1421)

Signed-off-by: mathetake <takeshi@tetrate.io>

compileopts/target.go

@@ -55,6 +55,7 @@ type TargetSpec struct {
 	JLinkDevice      string   `json:"jlink-device"`
 	CodeModel        string   `json:"code-model"`
 	RelocationModel  string   `json:"relocation-model"`
+	WasmAbi          string   `json:"wasm-abi"`
 }
 
 // overrideProperties overrides all properties that are set in child into itself using reflection.