[libunwind] Minor fixes in libunwind

 * When _LIBUNWIND_SUPPORT_COMPACT_UNWIND is defined in config.h,
   define it to "1" like the other macros. These macros are still
   checked using "#if defined(...)", however.

 * Include libunwind.h in AddressSpace.hpp before using
   _LIBUNWIND_ARM_EHABI.

 * Rename ProcessFrameHeaderCache to TheFrameHeaderCache, because some
   configurations (e.g. Android / hermetic static libraries) can have
   one cache per shared object in the process. (When there are more
   copies, it's more important not to waste memory in the cache.)

 * Add 3 missing header files to LIBUNWIND_HEADERS.

Differential Revision: https://reviews.llvm.org/D86254