Use simarm_x64 when targeting arm (#10010)

* GN changes for simarm_x64

* GN changes for simarm_x64

* Cleanup a bit

* Guard simarm_x64 behind a flag

* Remove flag

* Remove Mac specificity

tools/gn

@@ -201,10 +201,9 @@ def to_gn_args(args):
 
     gn_args['dart_custom_version_for_pub'] = 'flutter'
 
-    # Make sure host_cpu matches the bit width of target_cpu.
-    target_is_32_bit = gn_args['target_cpu'] == 'arm' or gn_args['target_cpu'] == 'x86'
-    if target_is_32_bit:
-      gn_args["host_cpu"] = "x86"
+    # Make sure host_cpu matches the bit width of target_cpu on x86.
+    if gn_args['target_cpu'] == 'x86':
+      gn_args['host_cpu'] = 'x86'
 
     gn_args['flutter_runtime_mode'] = runtime_mode