[libcxx] Link against android_support when needed

libc++ on Android needs to be linked against libandroid_support on API
levels less than 21 to provide needed functions that aren't in the libc
on those platforms (e.g. posix_memalign for libcxxabi). libc++ from the
NDK is a linker script that pulls in libandroid_support, but for
building libc++ itself, we need to explicitly add libandroid_support as
a dependency. Moreover, libc++ headers reference the functions provided
by libandroid_support, so it needs to be added as a public dependency.

Differential Revision: https://reviews.llvm.org/D73516

libcxx/CMakeLists.txt

@@ -779,6 +779,10 @@ function(cxx_link_system_libraries target)
     # (e.g. `printfw`/`scanfw`)
     target_link_libraries(${target} PRIVATE iso_stdio_wide_specifiers)
   endif()
+
+  if (ANDROID AND ANDROID_PLATFORM_LEVEL LESS 21)
+    target_link_libraries(${target} PUBLIC android_support)
+  endif()
 endfunction()
 
 # Windows-related flags =======================================================

libcxxabi/src/CMakeLists.txt

@@ -95,6 +95,9 @@ if (MINGW)
   # MINGW_LIBRARIES is defined in config-ix.cmake
   list(APPEND LIBCXXABI_LIBRARIES ${MINGW_LIBRARIES})
 endif()
+if (ANDROID AND ANDROID_PLATFORM_LEVEL LESS 21)
+  list(APPEND LIBCXXABI_LIBRARIES android_support)
+endif()
 
 if (NOT LIBCXXABI_USE_COMPILER_RT)
   add_library_flags_if(LIBCXXABI_HAS_GCC_LIB gcc)