gallium/util: Android backtrace support

gallium/util: Android backtrace support

We can't use any of the existing implementations in u_debug_stack.
Android technically has libunwind, but it's been modified to the point
where it no longer compiles with the Mesa usage. The library is also
not meant to be referenced by vendor libraries. The officially sanctioned
way of obtaining backtraces is through the Android own libbacktrace, a
C++ library. Access it through a separate C++ source file on Android only.

Signed-off-by: Stefan Schake <stschake@gmail.com>
Acked-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>