Fix ARM bootstrap failure due to an odd warning:

/src/gcc/gcc/config/arm/arm-builtins.c: In function 'rtx_def* arm_expand_builtin(tree, rtx, rtx, machine_mode, int)':
/src/gcc/gcc/config/arm/arm-builtins.c:3056:46: error: type qualifiers ignored on cast result type [-Werror=ignored-qualifiers]
     if (d->code == (const enum arm_builtins) fcode)
                                              ^~~~~

Avoid the warning by removing const, and bootstrap is OK again.

Committed as trivial patch.

    gcc/
	* config/arm/arm-builtins.c (arm_expand_builtin): Remove const.

From-SVN: r248686