Backport PR libstdc++/65142 fix from mainline

	PR libstdc++/65142
	* src/c++11/random.cc (random_device::_M_getval()): Check read result
	and retry after short reads.

Index: libstdc++-v3/src/c++11/random.cc
===================================================================
--- libstdc++-v3/src/c++11/random.cc	(revision 228467)
+++ libstdc++-v3/src/c++11/random.cc	(working copy)
@@ -30,6 +30,11 @@
 # include <cpuid.h>
 #endif
 
+#include <cerrno>
+
+#ifdef _GLIBCXX_HAVE_UNISTD_H
+# include <unistd.h>
+#endif
 
 namespace std _GLIBCXX_VISIBILITY(default)
 {
@@ -126,8 +131,27 @@ namespace std _GLIBCXX_VISIBILITY(defaul
 #endif
 
     result_type __ret;
-    std::fread(reinterpret_cast<void*>(&__ret), sizeof(result_type),
-	       1, _M_file);
+    void* p = &__ret;
+    size_t n = sizeof(result_type);
+#ifdef _GLIBCXX_HAVE_UNISTD_H
+    do
+      {
+	const int e = read(fileno(_M_file), p, n);
+	if (e > 0)
+	  {
+	    n -= e;
+	    p = static_cast<char*>(p) + e;
+	  }
+	else if (e != -1 || errno != EINTR)
+	  __throw_runtime_error(__N("random_device could not be read"));
+      }
+    while (n > 0);
+#else
+    const size_t e = std::fread(p, n, 1, _M_file);
+    if (e != 1)
+      __throw_runtime_error(__N("random_device could not be read"));
+#endif
+
     return __ret;
   }