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
diff --git a/gsi_x86.mk b/gsi_x86.mk
index ca88bed..0dfde63 100644
--- a/gsi_x86.mk
+++ b/gsi_x86.mk
@@ -44,4 +44,4 @@
 PRODUCT_NAME := gsi_x86
 PRODUCT_DEVICE := generic_x86
 PRODUCT_BRAND := Android
-PRODUCT_MODEL := x86 GSI
+PRODUCT_MODEL := GSI on x86
diff --git a/gsi_x86_64.mk b/gsi_x86_64.mk
index d2e3b3a..eb5bd97 100644
--- a/gsi_x86_64.mk
+++ b/gsi_x86_64.mk
@@ -45,4 +45,4 @@
 PRODUCT_NAME := gsi_x86_64
 PRODUCT_DEVICE := generic_x86_64
 PRODUCT_BRAND := Android
-PRODUCT_MODEL := x86_64 GSI
+PRODUCT_MODEL := GSI on x86_64