Initialize callTgt variable

This silcences a clang build failure:
dalvik/vm/compiler/codegen/arm/armv7-a-neon/../CodegenDriver.cpp:904:41:
error: variable 'callTgt' is uninitialized when used here
[-Werror,-Wuninitialized]
        LOAD_FUNC_ADDR(cUnit, r2, (int) callTgt);
                                        ^~~~~~~

Change-Id: Ic5b3bb492f19b842743bf7d6214c89301ff524e7
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>