[Backtracing][Linux] Disable invoking process_vm_readv() for older Android APIs

[finagolfin]

The Android community CI has been broken ever since this was merged in #66334 last month, because that CI builds against Android API 21:

/home/ubuntu/jenkins/workspace/oss-swift-RA-linux-ubuntu-16.04-android-arm64/swift/stdlib/public/runtime/CrashHandlerLinux.cpp:665:12: error: use of undeclared identifier 'process_vm_readv'
    return process_vm_readv(memserver_pid, &local, 1, &remote, 1, 0);
           ^
1 error generated.

Disable invoking this function before API 23, where it was added, which I confirmed gets the stdlib building with API 21 again.

@al45tair, please review.

[al45tair]

@swift-ci Please smoke test

[finagolfin]

@bnbarham, ready for merge.