bp2build: arch-configurable selects for label list attrs.

This CL adds the configurable LabelListAttribute support to bp2build.

Test: go test
Change-Id: I2ef9e385d9cf1b1845988128eca1d8cda1ecb5e8
diff --git a/bp2build/build_conversion_test.go b/bp2build/build_conversion_test.go
index b9b250a..ad88e97 100644
--- a/bp2build/build_conversion_test.go
+++ b/bp2build/build_conversion_test.go
@@ -502,8 +502,6 @@
 			expectedBazelTargets: []string{
 				`filegroup(
     name = "fg_foo",
-    srcs = [
-    ],
 )`,
 			},
 		},
@@ -1101,8 +1099,8 @@
         "out",
     ],
     srcs = [
-        "srcs-from-3",
         "in1",
+        "srcs-from-3",
     ],
 )`,
 			description: "genrule applies properties from genrule_defaults transitively",