Use gcc_fallthrough() instead of __attribute__((fallthrough)

Without this my build breaks on older compilers like GCC-5.4.

gcc/ChangeLog:

2017-03-13  Kito Cheng  <kito.cheng@gmail.com>

       * config/riscv/riscv.c (riscv_emit_float_compare>: Use
       gcc_fallthrough() instead of __attribute__((fallthrough));

From-SVN: r246116

gcc/ChangeLog

+2017-03-13  Kito Cheng  <kito.cheng@gmail.com>
+
+	* config/riscv/riscv.c (riscv_emit_float_compare>: Use
+	gcc_fallthrough() instead of __attribute__((fallthrough));
+
 2017-03-13  Gerald Pfeifer  <gerald@pfeifer.com>
 
 	* doc/gcc.texi: Remove "up" link to (DIR).

gcc/config/riscv/riscv.c

@@ -2089,13 +2089,13 @@ riscv_emit_float_compare (enum rtx_code *code, rtx *op0, rtx *op1)
 
     case UNLT:
       std::swap (cmp_op0, cmp_op1);
-      __attribute__((fallthrough));
+      gcc_fallthrough ();
 
     UNORDERED_COMPARISON(UNGT, le)
 
     case UNLE:
       std::swap (cmp_op0, cmp_op1);
-      __attribute__((fallthrough));
+      gcc_fallthrough ();
 
     UNORDERED_COMPARISON(UNGE, lt)
 #undef UNORDERED_COMPARISON