common.mk: use TARGET_GCC_VERSION for toolchain path

Summary: The build repo target makefiles ensure this environment variable is always set by the time the oprofile common makefile is inherited. Same as my AOSP submission here: https://android-review.googlesource.com/#/c/61766

Test Plan: Tested that the header path was correctly used in a build. Nothing more to test, carry on

Reviewers: michyprima, randall.rushing, rlasalle15, Mustaavalkosta

Differential Revision: http://review.codefirex.com/D181