simpleperf: switch to cpu-clock when cpu-cycles is missing on x86.

For https://github.com/android/ndk/issues/1558.

Also rename GetBuildArch() to GetTargetArch().

Bug: none
Test: run simpleperf manually
Test: run simpleperf_unit_test
Change-Id: I63a91777e2b2c210f42ca4032079297162b66f73
14 files changed