Add support for $(TARGET_EXPERIMENTAL_MTE) in product configs.

We are bringing up support for ARM Memory Tagging Extension in the platform,
but the required patches have not yet landed in the upstream kernel. This
change adds support for a product variable that products may set to true to
declare support for an experimental userspace interface to MTE. Setting the
variable to true causes a product configuration to be enabled which will
activate code in bionic and scudo that uses the interface.

This variable should not be set to true in shipping products because
the userspace interface has not been finalized and is subject to change.
Once the support patches have landed in the kernel, bionic and scudo will
be changed to use the official interface and the variable will be removed.

Bug: 135772972
Change-Id: I8ba5dea4de374eb6775edb6d4e3c0cf9cad8d760