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

Conflicts:
	liblog/logd_write.c