Use 'environ' C extern for environment variables (except Mac OS)

The function __fort_getenv uses the C extern
variable environ to get the values of the environment
variables. What's happening is 'environ' is getting
reallocated, but this code caches a value of 'environ'
in a local static pointer 'env' and dereferences
that. The old values in the old copy of 'environ'
that 'env' points to are no longer valid. The code
has been fixed using 'environ' directly rather than
using the local static pointer.

Author: gklimowicz

runtime/flang/initpar.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017, NVIDIA CORPORATION.  All rights reserved.
+ * Copyright (c) 1995-2018, NVIDIA CORPORATION.  All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -404,7 +404,11 @@ __fort_getenv(const char *nm)
   int n;
 
   n = strlen(nm);
+#if defined(TARGET_OSX)
   e = env;
+#else             
+  e = environ;
+#endif
   while (*e != NULL) {
     if ((strncmp(*e, nm, n) == 0) && ((*((*e) + n)) == '=')) {
       return ((*e) + n + 1);