Make arm_on_x86 symmetric

We only define arm_on_x86 in the x86 code, but sometimes arm code needs
to know that it's working in the emulated mode, too.

Test: CtsRsCppTestCases

Bug: b/75971275

Change-Id: I99564fbe9aeb284e2f11ffb593b18536a7755ea5
(cherry picked from commit 5eb8ec1e)