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/crashdump.c b/microdroid/kdump/crashdump.c
index 7f892f9..a606d43 100644
--- a/microdroid/kdump/crashdump.c
+++ b/microdroid/kdump/crashdump.c
@@ -80,7 +80,7 @@
     if (fstat(vmcore, &statbuf) == -1) {
         FAIL("Failed to stat %s", DUMP_SOURCE);
     }
-    printf("Size is %ld bytes\n", (long)statbuf.st_size);
+    printf("Size is %ld bytes\n", statbuf.st_size);
 
     // sendfile(2) is faster, can't be used because /proc/vmcore doesn't support splice_read
     size_t dumped = 0;