Build kdump only for 64-bit

It doesn't support 32-bit target. Build crashdump, kexec, and the kernel
only for 64-bit and disable the 32-bit variant.

Bug: N/A
Test: build for aosp_x86 and full
Change-Id: I3fafff4a1a45ab808aa3b8cd720925c6b8f4fef3
diff --git a/microdroid/kdump/Android.bp b/microdroid/kdump/Android.bp
index ffbfcce..390886b 100644
--- a/microdroid/kdump/Android.bp
+++ b/microdroid/kdump/Android.bp
@@ -7,6 +7,7 @@
     stem: "kexec",
     srcs: ["kexec.c"],
     installable: false,
+    compile_multilib: "64",
 }
 
 cc_binary {
@@ -15,11 +16,16 @@
     srcs: ["crashdump.c"],
     static_executable: true,
     installable: false,
+    compile_multilib: "64",
 }
 
 android_filesystem {
     name: "microdroid_crashdump_initrd",
-    deps: ["microdroid_crashdump"],
+    multilib: {
+        lib64: {
+            deps: ["microdroid_crashdump"],
+        },
+    },
     dirs: [
         "dev",
         "proc",