simpleperf: find tools in clang prebuilts.

When running simpleperf in AOSP projects, search llvm tools from clang
prebuilts. Because It usually has a newer version than tools in ndk.

Bug: none
Test: run test/test.py TestTools* and print tool paths.
Change-Id: Icd5ba72bc82800aebb1d6f378f8088ed19b86ac0
1 file changed