Fix arm build failure with libgcc

[zatrazz]

Summary

Both arm32 armv7/armv8 bots are failing to build due to a linking
issue:

[100%] Built target cxxabi_static
CMakeFiles/cxxabi_shared.dir/cxa_demangle.cpp.o: In function `(anonymous namespace)::itanium_demangle::OutputStream::writeUnsigned(unsigned long long, bool)':
/home/buildslave/buildslave/libcxx-libcxxabi-libunwind-armv7-linux-noexceptions/llvm/projects/libcxxabi/src/demangle/Utility.h:55: undefined reference to `aeabi_uldivmod'
/home/buildslave/buildslave/libcxx-libcxxabi-libunwind-armv7-linux-noexceptions/llvm/projects/libcxxabi/src/demangle/Utility.h:56: undefined reference to `aeabi_uldivmod'
clang-6.0: error: linker command failed with exit code 1 (use -v to see invocation)

It seems after r371273 OutputStream is used more extensively and
is pulling OutputStream::writeUnsigned (which thus requires unsigned
integer module).

The straightfoward fix is to explicit link against libgcc if
compiler-rt is not used.

[zatrazz]

Ping.

[rovka]

I'm not very familiar with this area, but it seems like a sensible fix.

libcxxabi/cmake/config-ix.cmake

 include(CMakePushCheckState)
 include(CheckLibraryExists)
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
 include(CheckCSourceCompiles)
 
 check_library_exists(c fopen "" LIBCXXABI_HAS_C_LIB)
 if (NOT LIBCXXABI_USE_COMPILER_RT)
   check_library_exists(gcc_s __gcc_personality_v0 "" LIBCXXABI_HAS_GCC_S_LIB)
+  check_library_exists(gcc __aeabi_uldivmod "" LIBCXXABI_HAS_GCC_LIB)
 endif ()
 
 # libc++abi is built with -nodefaultlibs, so we want all our checks to also
 # use this option, otherwise we may end up with an inconsistency between
 # the flags we think we require during configuration (if the checks are
 # performed without -nodefaultlibs) and the flags that are actually
 # required during compilation (which has the -nodefaultlibs). libc is
 # required for the link to go through. We remove sanitizers from the
 # configuration checks to avoid spurious link errors.
 check_c_compiler_flag(-nodefaultlibs LIBCXXABI_HAS_NODEFAULTLIBS_FLAG)
 if (LIBCXXABI_HAS_NODEFAULTLIBS_FLAG)
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nodefaultlibs")
   if (LIBCXXABI_HAS_C_LIB)
     list(APPEND CMAKE_REQUIRED_LIBRARIES c)
   endif ()
   if (LIBCXXABI_USE_COMPILER_RT)
     list(APPEND CMAKE_REQUIRED_FLAGS -rtlib=compiler-rt)
     find_compiler_rt_library(builtins LIBCXXABI_BUILTINS_LIBRARY)
     list(APPEND CMAKE_REQUIRED_LIBRARIES "${LIBCXXABI_BUILTINS_LIBRARY}")
-  elseif (LIBCXXABI_HAS_GCC_S_LIB)
+  else ()
+    if (LIBCXXABI_HAS_GCC_S_LIB)
       list(APPEND CMAKE_REQUIRED_LIBRARIES gcc_s)
     endif ()
+    if (LIBCXXABI_HAS_GCC_LIB)
+      list(APPEND CMAKE_REQUIRED_LIBRARIES gcc)
+    endif ()
+  endif ()
   if (MINGW)
     # Mingw64 requires quite a few "C" runtime libraries in order for basic
     # programs to link successfully with -nodefaultlibs.
     if (LIBCXXABI_USE_COMPILER_RT)
       set(MINGW_RUNTIME ${LIBCXXABI_BUILTINS_LIBRARY})
     else ()
       set(MINGW_RUNTIME gcc_s gcc)
     endif()

libcxxabi/src/CMakeLists.txt

 else()
   add_library_flags_if(LIBCXXABI_HAS_GCC_S_LIB gcc_s)
 endif()
 if (MINGW)
   # MINGW_LIBRARIES is defined in config-ix.cmake
   list(APPEND LIBCXXABI_LIBRARIES ${MINGW_LIBRARIES})
 endif()
 
+if (NOT LIBCXXABI_USE_COMPILER_RT)
+  add_library_flags_if(LIBCXXABI_HAS_GCC_LIB gcc)
+endif ()
+
 # Setup flags.
 add_link_flags_if_supported(-nodefaultlibs)
 
 set(LIBCXXABI_SHARED_LINK_FLAGS)
 
 if ( APPLE )
   if ( CMAKE_OSX_DEPLOYMENT_TARGET STREQUAL "10.6" )
     list(APPEND LIBCXXABI_COMPILE_FLAGS "-U__STRICT_ANSI__")