Add -fprofile-sample-accurate flag for sampling PGO

This flag indicates that the sample profile data matches the source
and can be fully trusted. The compiler will make aggressive
assumption that functions without any samples are cold functions,
and will optimize for size for them.

Test: binary size reduces to match instr PGO
Bug: 79161490
Change-Id: I53d6d05be70c39e5eb28b2f5b0549d9eb6b5cc62