Import ctzdi2.c from LLVM compiler-rt

Import ctzdi2.c from LLVM compiler-rt

When using __builtin_ctzll() in AArch32 code, the compiler may translate
that into a call to the __ctzdi2() function. In this case, the linking
phase fails because TF doesn't provide an implementation for it.

This patch imports the implementation of the __ctzdi2() function from
LLVM's compiler-rt project and hooks it into TF's build system. The
ctzdi2.c file is an unmodified copy from the master branch as of
July 19 2017 (SVN revision: 308480).

Change-Id: I96766a025ba28e1afc6ef6a5c4ef91d85fc8f32b
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>