liblog: add required function

Some Motorola proprietaries require a function named __android_log_loggable. This
change adds the function and solves problems with radio and other
binaries on qualcomm based Motorola phones. This code is activated with
TARGET_USES_MOTOROLA_LOG:=true

Change-Id: I722b42d34198c027f265c3cf9f220d9798814915
Merge commit 'refs/changes/34/76634/1' of http://review.cyanogenmod.org/CyanogenMod/android_system_core into 5.0