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: I9837c42e6bab38a894ece4e0f34b64f81b38fef0


[ROCm/clr commit: f7cc49c0b2]

projects/clr/hipamd/lpl_ca/pstreams/pstream.h

@@ -70,12 +70,12 @@ struct pstreams {
     /// Type used for file descriptors.
     typedef int fd_type;
 
-    static const pmode pstdin = std::ios_base::out;   ///< Write to stdin
-    static const pmode pstdout = std::ios_base::in;   ///< Read from stdout
-    static const pmode pstderr = std::ios_base::app;  ///< Read from stderr
+    static constexpr pmode pstdin = std::ios_base::out;   ///< Write to stdin
+    static constexpr pmode pstdout = std::ios_base::in;   ///< Read from stdout
+    static constexpr pmode pstderr = std::ios_base::app;  ///< Read from stderr
 
     /// Create a new process group for the child process.
-    static const pmode newpg = std::ios_base::trunc;
+    static constexpr pmode newpg = std::ios_base::trunc;
 
    protected:
     enum { bufsz = 32 };  ///< Size of pstreambuf buffers.