Update profile_file discovery rules

Bug: http://b/63768402

Profile files are searched in multiple directories in a pre-define list,
instead of just one.

PGO profile_file enforcement is also removed - profile_file dependency
and -fprofile-use are added iff the profile_file is present.

Test: Build https://android-review.googlesource.com/c/474805 with
.profdata files in vendor/google_data/pgo-profiles and
toolchain/pgo-profiles and verify expected behavior.

Change-Id: I2604713a8a7a682bd5c6bde569dc32335be5eca9
Merged-In: I2604713a8a7a682bd5c6bde569dc32335be5eca9