llvm: Add workaround for STLport not being a full C++11 STL

llvm checks for C++11 support in the compiler, and assumes the
STL has full C++11 support if the compiler does.
However, for the combination of gcc 4.7 and STLport, that assumption
is incorrect -- the compiler supports rvalue references just fine, but
the STL hasn't caught up yet.

Longer term, the better fix would be to fix STLport.

Change-Id: I20237a19a3835faee583f429749751ee3c4843f6
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>