libbcc: Don't implement __ashldi3 and __lshrdi3 with current gcc versions

Current gcc versions (4.6, 4.7) complain about duplicate definitions
of __ashldi3 and __lshrdi3 (defined here and in libgcc) at -O3

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