V8: avoid deadlock when profiling is active

A deadlock happens when sampler initiated by SIGPROF tries to lock
the thread and the thread is already locked by the same thread. As
a result, other thread involved in sampling process hangs. The
patch adds a check for thread lock before continuing sampler
operation.

The fix has been tested on a sample app under load with and without
profiling turned on.

Fixes issue #14576 and specifically the duplicate issue #25295

Reviewed-By: Julien Gilli <julien.gilli@joyent.com>
PR-URL: https://github.com/joyent/node/pull/25309