Android's native bridge (i.e. AArch64 emulator) doesn't support TBI so
we need a way to disable TBI on Linux when targeting the native bridge.
This can also be used to test the no-TBI code path on Linux (currently
only used on Fuchsia), or make Scudo compatible with very old
(pre-commit d50240a5f6ceaf690a77b0fccb17be51cfa151c2 from June 2013)
Linux kernels that do not enable TBI.