core: Support inline PGO profile generation

Summary:
Support setting -fprofile-instr-generate if BUILD_PGO_INSTRUMENT is set.
This is a newer addition to LLVM CI and no longer requires perf
profiling (either directly or via my ALAPerf wrapper) for instrumentation.

Test Plan: gtg

Reviewers: randall.rushing, anders3408, jriordan001, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D427