Pass -Wno-backend-plugin when using profiles

Bug: http://b/63768402

Wno-backend-plugin warnings are generated by IR-based PGO when sources
evolve and the profiles become stale.  Disable these warnings when
profiles are passed to the compiler.

Test: Build a module with a PGO property when profiles are used.
Change-Id: I462bbf55aafd8770a90c2c23462bf71607ac9108