Search for PGO profiles in PGO_ADDITIONAL_PROFILE_DIRS

This variable can be set in BoardConfig.mk to specify a list of
additional paths that contain PGO profiles.  These directories are
searched after the predefined paths in soong/cc/pgo.go while finding
PGO profiles.

Test: Set this variable in a BoardConfig and verify that such profiles
are found and that these paths are searched after the predefined paths
in soong/cc/pgo.go.

Change-Id: I0bb9523de614d0f23aba8d51c887d8fc8f41c993