rust: Use prebuilts for x86 host target.

Use prebuilts for x86 host targets to make both x86 and x86_64 builds
more consistent with one another. This fixes building x86 binaries.

Bug: 162063992
Bug: 169347277
Test: Build 32-bit host module.
Change-Id: Id11eb0f3b949439cec090060a33b755af94b5883

rust/testing.go

@@ -45,6 +45,30 @@ func GatherRequiredDepsForTest() string {
 				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",