rust: Support new rust_stdlib_prebuilt_host type

Refactor Rust prebuilts to support the new rust_stdlib_prebuilt_host
module type, and change the format for depending on the prebuilt host
stdlibs.

Bug: 140642453
Test: m
Change-Id: Ifbc4741818777934e917631c788b20911856c44a
diff --git a/rust/testing.go b/rust/testing.go
index 94cdd9d..dac04d6 100644
--- a/rust/testing.go
+++ b/rust/testing.go
@@ -53,74 +53,14 @@
 func GatherRequiredDepsForTest() string {
 	bp := `
 		rust_prebuilt_library {
-				name: "libstd_x86_64-unknown-linux-gnu",
-                                crate_name: "std",
-                                rlib: {
-                                    srcs: ["libstd.rlib"],
-                                },
-                                dylib: {
-                                    srcs: ["libstd.so"],
-                                },
-				host_supported: true,
-				sysroot: true,
-		}
-		rust_prebuilt_library {
-				name: "libtest_x86_64-unknown-linux-gnu",
-                                crate_name: "test",
-                                rlib: {
-                                    srcs: ["libtest.rlib"],
-                                },
-                                dylib: {
-                                    srcs: ["libtest.so"],
-                                },
-				host_supported: true,
-				sysroot: true,
-		}
-		rust_prebuilt_library {
-				name: "libstd_i686-unknown-linux-gnu",
-                                crate_name: "std",
-                                rlib: {
-                                    srcs: ["libstd.rlib"],
-                                },
-                                dylib: {
-                                    srcs: ["libstd.so"],
-                                },
-				host_supported: true,
-				sysroot: true,
-		}
-		rust_prebuilt_library {
-				name: "libtest_i686-unknown-linux-gnu",
-                                crate_name: "test",
-                                rlib: {
-                                    srcs: ["libtest.rlib"],
-                                },
-                                dylib: {
-                                    srcs: ["libtest.so"],
-                                },
-				host_supported: true,
-				sysroot: true,
-		}
-		rust_prebuilt_library {
-				name: "libstd_x86_64-apple-darwin",
-                                crate_name: "std",
-                                rlib: {
-                                    srcs: ["libstd.rlib"],
-                                },
-                                dylib: {
-                                    srcs: ["libstd.so"],
-                                },
-				host_supported: true,
-				sysroot: true,
-		}
-		rust_prebuilt_library {
-				name: "libtest_x86_64-apple-darwin",
-                                crate_name: "test",
-                                rlib: {
-                                    srcs: ["libtest.rlib"],
-                                },
-                                dylib: {
-                                    srcs: ["libtest.so"],
-                                },
+				name: "libstd",
+				crate_name: "std",
+				rlib: {
+					srcs: ["libstd.rlib"],
+				},
+				dylib: {
+					srcs: ["libstd.so"],
+				},
 				host_supported: true,
 				sysroot: true,
 		}