[libcxxabi] Fix arm build failer with libgcc

Both arm32 armv7/armv8 bots which do not use compiler-rt are failing
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.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@372921 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed