We need to rebuild clang with libc++ to get compatible headers for cross
builds. libc++abi is a dependency of libc++, as the build instructions
says [0].
[0] http://libcxx.llvm.org/docs/BuildingLibcxx.html
Instead, copy libgcc from the GCC used to build clang at each stage.
When passing --gcc-toolchain, the flag ends up appearing in the output
of llvm-config, and completely defeats the purpose of copying libgcc in
clang/lib/gcc.
Since build-clang.py requires a gcc_dir to be set, and we're using GCC
from there to build clang, we might as well copy libgcc from there
instead of building a fresh GCC. On the taskcluster job building clang,
GCC comes from a tooltool package that is already the result of
build-gcc anyways.
When passing -DCMAKE_C_COMPILER="gcc -flags" to CMake, ninja then tries
to literally call "gcc -flags", not "gcc" "-flags", and a "gcc -flags"
file obviously doesn't exist, so the build fails.
This fixes a regression from bug 1042132. Hopefully, this also doesn't
regress bug 1042132 itself.
