Workaround TSan false positive in write fault handler

Workaround TSan false positive in write fault handler

The data races reported between GC_write_fault_handler,
catch_exception_raise and GC_install_header, GC_remove_header are OK
as the added or removed header should be already unprotected.

* os_dep.c [MPROTECT_VDB && THREADS] (is_header_found_async): New
static function (with GC_ATTR_NO_SANITIZE_THREAD attribute).
* os_dep.c [MPROTECT_VDB && !THREADS] (is_header_found_async): New
macro.
* os_dep.c [MPROTECT_VDB && !DARWIN] (GC_write_fault_handler): Use
is_header_found_async() instead of HDR(addr)!=0.
* os_dep.c [MPROTECT_VDB && DARWIN] (catch_exception_raise): Likewise.