ANDROID: add arch=arm64 explicitly.

This is the default, but we prefer to make it explicit.

Bug: 236012223
Change-Id: I754098d4d928a61c042a310ee43833ca17847dca
Signed-off-by: HONG Yifan <elsk@google.com>

BUILD.bazel

@@ -398,6 +398,7 @@ kernel_build(
         "vmlinux",
         "vmlinux.symvers",
     ],
+    arch = "arm64",
     build_config = "build.config.common",
     defconfig = "arch/arm64/configs/microdroid_defconfig",
     make_goals = [
@@ -432,6 +433,7 @@ kernel_build(
         "vmlinux",
         "vmlinux.symvers",
     ],
+    arch = "arm64",
     build_config = "build.config.common",
     defconfig = "arch/arm64/configs/microdroid_defconfig",
     make_goals = [
@@ -961,6 +963,7 @@ kernel_build(
         "vmlinux",
         "vmlinux.symvers",
     ],
+    arch = "arm64",
     build_config = "build.config.rockpi4",
     defconfig = "arch/arm64/configs/gki_defconfig",
     dtstree = "//common-modules/virtual-device:rockpi4_dts",
@@ -988,6 +991,7 @@ kernel_build(
         "vmlinux",
         "vmlinux.symvers",
     ],
+    arch = "arm64",
     build_config = "build.config.rockpi4",
     dtstree = "//common-modules/virtual-device:rockpi4_dts",
     make_goals = [
@@ -1268,6 +1272,7 @@ kernel_build(
     # Hack to actually check the build.
     # Otherwise, Bazel thinks that there are no output files, and skip building.
     outs = [".config"],
+    arch = "arm64",
     build_config = "build.config.common",
     defconfig = "//build/kernel/kleaf:allmodconfig",
     make_goals = [