Add cross target to CC if using clang. Also add custom gcc toolchain
path for fallback gcc tools.

Clang will fallback to using things like ld, as, and libgcc if
(respectively) one of the llvm linkers isn't available, the integrated
assembler is turned off, or an appropriately cross-compiled version of
compiler-rt isn't available. To this end, you can specify the path to
this fallback gcc toolchain with GCC_TOOLCHAIN.
Signed-off-by: NBehan Webster <behanw@converseincode.com>
Reviewed-by: NJan-Simon Möller <dl9pf@gmx.de>
Reviewed-by: NMark Charlebois <charlebm@gmail.com>
Signed-off-by: NGreg Hackmann <ghackmann@google.com>
Signed-off-by: NMatthias Kaehlcke <mka@chromium.org>
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>