Enable com.android.support.apexer only for linux_glibc_x86_64 target

With the addition of linux_bionic_arm64 host target, this test apex
automatically gets enabled for the cross host target. However, some
dependencies of this apex like soong_zip, etc. are not available for the
cross-host target.  As a result, we get the following errors:

error: system/apex/apexer/Android.bp:104:1: dependency "merge_zips" of
"com.android.support.apexer" missing variant:
  os:linux_bionic, arch:arm64
available variants:
  os:linux_glibc, arch:x86_64
error: system/apex/apexer/Android.bp:104:1: dependency "soong_zip"
of "com.android.support.apexer" missing variant:
  os:linux_bionic, arch:arm64
available variants:
  os:linux_glibc, arch:x86_64

Fix the problem by disabling this apex for targets where the tools are
not available.

Bug: 159685774
Test: HOST_CROSS_OS=linux_bionic HOST_CROSS_ARCH=arm64 m

Change-Id: Ie79b116438da3b08d3aba65391affa409a3dfa36
diff --git a/apexer/Android.bp b/apexer/Android.bp
index 2953754..03fdbd6 100644
--- a/apexer/Android.bp
+++ b/apexer/Android.bp
@@ -110,6 +110,12 @@
   ignore_system_library_special_case: true,
   key: "com.android.support.apexer.key",
   binaries: apexer_tools + apexer_go_tools,
+  enabled: false,
+  target: {
+    linux_glibc_x86_64: {
+      enabled: true,
+    },
+  },
 }
 
 // TODO(b/148659029): this test can't run in TEST_MAPPING.