Make gas ignore the ".set hardfloat" directive on mips64.

The directive is needed in certain parts of the kernel code when
compiling using clang. By ignoring the directive, the old toolchain
will still be able to build the kernel after the code has been adjusted
for clang.

OK kettenis@

gnu/usr.bin/binutils-2.17/gas/config/tc-mips.c

@@ -12355,6 +12355,8 @@ s_mipsset (int x ATTRIBUTE_UNUSED)
     mips_opts.sym32 = TRUE;
   else if (strcmp (name, "nosym32") == 0)
     mips_opts.sym32 = FALSE;
+  else if (strcmp (name, "hardfloat") == 0)
+    /* ignored */;
   else
     {
       as_warn (_("Tried to set unrecognized symbol: %s\n"), name);