Fixing compilation errors related to MUSL libc

Fix Musl libc NULL errors and unsupported pthread funcs for compatibility.
Also ensures cleanup and error handling irrespective of CPU affinity override.

Fix submitted by github dev - AngryLoki
https://github.com/ROCm/ROCR-Runtime/issues/181

Change-Id: Ia487315e504112be5d3370756f23f6e23b9ae4be

runtime/hsa-runtime/core/runtime/default_signal.cpp

@@ -57,7 +57,7 @@ int BusyWaitSignal::rtti_id_ = 0;
 BusyWaitSignal::BusyWaitSignal(SharedSignal* abi_block, bool enableIPC)
     : Signal(abi_block, enableIPC) {
   signal_.kind = AMD_SIGNAL_KIND_USER;
-  signal_.event_mailbox_ptr = NULL;
+  signal_.event_mailbox_ptr = uint64_t(NULL);
 }
 
 hsa_signal_value_t BusyWaitSignal::LoadRelaxed() {