gn build: Support cross-compiling libunwind for Android.

- Usual cross-compilation fix: s/target_/current_/g

- Define _LIBUNWIND_IS_NATIVE_ONLY to enable unwinding past
  functions with return pointer authentication.

- Android needs two libunwind static libraries: one with symbols exported and
  one without. These both need to be in the same build tree so
  the libunwind_hermetic_static_library configuration option doesn't
  help here. Replace it with build rules that build both libraries.

- Install the libraries in the location that Android expects them to be.

Differential Revision: https://reviews.llvm.org/D96563