Fix unit test by holding compiler_lock_ on ourselves.
Fix unit test by holding compiler_lock_ on ourselves.

Because in previous CL, we removed the MutexLock in the
Materialize for parallel compilation, we don't hold the
lock before we call LoadElfFromCompilationUnit anymore.
Thus, we have to acquire for the lock in
LoadElfFromCompilationUnit.

Change-Id: Ia1b059bdffafc54a473820244bcfcfc634771061

treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst