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
2 files changed