[libunwind][mips] Include gcc_s for linkage

[smaksimovic]

Summary

When compiling with optimizations, mips requires various helper routines(__ashldi3 and the like) contained in libgcc_s.
Conditionally include libgcc_s in the set of libraries to be linked to.

[atanasyan]

LGTM from MIPS point of view.

[joerg]

This hard-coding seems to be counter-productive, it could be compiler-rt just as well.

[smaksimovic]

@joerg Would it be okay to commit this?
The LIBUNWIND_HAS_GCC_S_LIB although declared, did not have its value used anywhere unlike LIBUNWIND_HAS_C_LIB and LIBUNWIND_HAS_DL_LIB which had been declared/used similarly.
The added line is only executed if the following block is (cmake/config-ix.cmake):

if (NOT LIBUNWIND_USE_COMPILER_RT)
  check_library_exists(gcc_s __gcc_personality_v0 "" LIBUNWIND_HAS_GCC_S_LIB)
endif()

in which case compiler-rt is not being used anyway, if that's what you were referring to.

[joerg]

If a build against compiler-rt works, that's ok. It wasn't clear from the diff.

src/CMakeLists.txt

 set(LIBUNWIND_SOURCES
     ${LIBUNWIND_CXX_SOURCES}
     ${LIBUNWIND_C_SOURCES}
     ${LIBUNWIND_ASM_SOURCES})
 
 # Generate library list.
 set(libraries ${LIBUNWINDCXX_ABI_LIBRARIES})
 append_if(libraries LIBUNWIND_HAS_C_LIB c)
+append_if(libraries LIBUNWIND_HAS_GCC_S_LIB gcc_s)
 append_if(libraries LIBUNWIND_HAS_DL_LIB dl)
 if (LIBUNWIND_ENABLE_THREADS)
   append_if(libraries LIBUNWIND_HAS_PTHREAD_LIB pthread)
 endif()
 
 # Setup flags.
 append_if(LIBUNWIND_CXX_FLAGS LIBUNWIND_HAS_NO_RTTI_FLAG -fno-rtti)