gn2bp: Only compile x64 for host machines

* The generated desc_*.json files except the compiling(not target) cpu architecture to be x64. which
  was an issue because Soong tries to compile for both x32 and x64.

Test: mma
Change-Id: I6be5ed045a25e4020e69bf0157f919f43e68e2c0
diff --git a/tools/gn2bp/gen_android_bp b/tools/gn2bp/gen_android_bp
index a0e719d..c339664 100755
--- a/tools/gn2bp/gen_android_bp
+++ b/tools/gn2bp/gen_android_bp
@@ -327,6 +327,9 @@
     self.generated_headers = set()
     self.export_generated_headers = set()
     self.ldflags = set()
+    self.compile_multilib = None
+    if name == 'host':
+      self.compile_multilib = '64'
 
   def to_string(self, output):
     nested_out = []
@@ -347,6 +350,9 @@
     self._output_field(nested_out, 'ldflags')
 
     if nested_out:
+      # This is added here to make sure it doesn't add a `host` arch-specific module just for
+      # `compile_multilib` flag.
+      self._output_field(nested_out, 'compile_multilib')
       output.append('    %s: {' % self.name)
       for line in nested_out:
         output.append('    %s' % line)