Convert FALLTHROUGH comments to C++17 [[fallthrough]] attribute

Use conditionals and a macro to avoid provoking compilers that do
not support this attribute.

Adds a macro named CPP_ATTRIBUTE_FALLTHROUGH which is invoked right
before a following case statement to declare that the intent is to
fall through.  Example:

    ...
    case xxx:
       ...
       CPP_ATTRIBUTE_FALLTHROUGH;
    case yyy:
    ...

The gcc/clang alternative of adding comments that contain "FALLTHROUGH"
suffers from the problem that *by default* ccache strips the comments
so that they are not present for the real compilation.

Change-Id: I77ddeb7dae46db8398b014a93f6a71bedc64ada9
Signed-off-by: Dale Stimson <dale.b.stimson@intel.com>

runtime/helpers/hash.h

@@ -21,6 +21,7 @@
  */
 
 #pragma once
+#include "common/compiler_support.h"
 #include "runtime/helpers/aligned_memory.h"
 #include <cstdint>
 
@@ -51,11 +52,11 @@ class Hash {
         case 3:
             value = static_cast<uint32_t>(*reinterpret_cast<const unsigned char *>(data++));
             value <<= 8;
-        // FALLTHROUGH
+            CPP_ATTRIBUTE_FALLTHROUGH;
         case 2:
             value |= static_cast<uint32_t>(*reinterpret_cast<const unsigned char *>(data++));
             value <<= 8;
-        // FALLTHROUGH
+            CPP_ATTRIBUTE_FALLTHROUGH;
         case 1:
             value |= static_cast<uint32_t>(*reinterpret_cast<const unsigned char *>(data++));
             value <<= 8;