Add -D__ANDROID_NATIVE_BRIDGE__ to native_bridge targets

And remove dual meaning from arm_on_x86 target. It was set for x86
targets with arm support and for arm targets with x86 support - and that
2nd one was abused for native_bridge version of RenderScript.

It's much safer and cleaner to rely on __ANDROID_NATIVE_BRIDGE__ there
while leaving arm_on_x86 to mean "x86 binary built in the image with arm
support".

It's only used by bcc compiler which is in canadian-cross situation here:
it's pure x86-64 binary, but it needs to know about ALL supported
architectures on device because it needs to include appropriate codegen
module.

Bug: http://b/153609531

Test: cts-tradefed run cts --abi armeabi-v7a -m CtsRsCppTestCases
Test: cts-tradefed run cts --abi arm64-v8a -m CtsRsCppTestCases

Change-Id: I869212e0b82eeaa30361a4e183d5c862ab40ef12