Do not use Q_GLOBAL_STATIC in the implementation of QMutex

Since Q_GLOBAL_STATIC might use QMutex and cause a stack overflow.

Task-number: QTBUG-47554
Change-Id: I4853c5e9b9168d4a417200e2a45a1bf9cb103a30
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Pierre Rossi <pierre.rossi@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>