i#3023: add new-thread barrier during attach

[derekbruening]

Adds a barrier to taken-over threads creating new threads until attach
is fully finished. This avoids problems keeping up with an app that
is creating threads while we try to attach.

Fixes #3023

[derekbruening]

run aarch64 tests

[derekbruening]

We have what I believe are flaky tests -- re-running (unclear how to re-run Jenkins build directly so did a new commit...will have to live w/ re-running Appveyor):

debug-internal-64: 163 tests passed, **** 2 tests failed, of which 1 were flaky: ****
	code_api|linux.signal1011 =>  Application /data/jenkins_root/workspace/DynamoRIO-AArch64-Precommit/build/build_debug-internal-64/suite/tests/bin/linux.signal1011 (13008).  Internal Error: DynamoRIO debug check failure: /data/jenkins_root/workspace/DynamoRIO-AArch64-Precommit/core/arch/mangle_shared.c:765 skip_pc != NULL  cti_pc != NULL 

debug-internal-64: 289 tests passed, **** 2 tests failed, of which 1 were flaky, but ignoring 0 for i: ****
	code_api|tool.drcacheoff.burst_threads 
	code_api|api.static_sideline_FLAKY 

[derekbruening]

I filed #3025 on the signal.1011 failure as it seems new (but did not reproduce)