Apply constexpr on global constant varaibles

When HIP_ENABLE_DEFERRED_LOADING=0, many global variables will be
referenced but they are not initialized in that early time. The patch
will use constexpr to initialze global constant varables in compile
time.

Change-Id: I9d538b7abc6a0ce700ec3332b97fc144db5fc1ef

rocclr/device/gpu/gpuprintf.cpp

@@ -236,8 +236,8 @@ int PrintfDbg::checkVectorSpecifier(const std::string& fmt, size_t startPos, siz
   return vectorSize;
 }
 
-static const size_t ConstStr = 0xffffffff;
-static const char Separator[] = ",\0";
+static constexpr size_t ConstStr = 0xffffffff;
+static constexpr char Separator[] = ",\0";
 
 size_t PrintfDbg::outputArgument(const std::string& fmt, bool printFloat, size_t size,
                                  const uint32_t* argument) const {