Consistent PRODUCT_MODEL name for x86(_64) GSI

The product model names of all other aosp_$arch and gsi_$arch are
of the form: "xxx on $arch".
So let's use the same format for gsi_x86(_64).

Bug: 177495405
Test: $ lunch gsi_x86-userdebug; get_build_var PRODUCT_MODEL
      GSI on x86
      $ lunch gsi_x86_64-userdebug; get_build_var PRODUCT_MODEL
      GSI on x86_64
Change-Id: I4c361600ce4f2a5b90cd22b79416e251803b09f0
2 files changed