Really allow overriding the LLVM prebuilts path.
Really allow overriding the LLVM prebuilts path.

Not sure why my previous testing was working without this, but kati is
a bit pickier about ?= vs :=.

Change-Id: I80243a3c93dec7fd9b8e6f92e2c6cc956706ace0

xorware/android_build,xorware/android_build,xorware/android_build,xorware/android_build,xorware/android_build