Fix m68k bootstrap due to -Wempty-body warning

Ah the jobs of conditional compilation...

A recent change is breaking m68k bootstrap as it doesn't have any
UNSPECV_VALUES and as a result we end up with

if (cond)
  ;
[ With no else clauses. ]

Which triggers:

> /home/jlaw/test/gcc/gcc/print-rtl.cc: In function ‘void print_exp(pretty_printer*, const_rtx, int)’:
> /home/jlaw/test/gcc/gcc/print-rtl.cc:1622:11: warning: suggest braces around empty body in an ‘if’ statement [-Wempty-body]

Rather than using ";" we can use a "{}" pair with proper formatting as the
warning suggests.

Committing as obvious and hopefully restoring m68k bootstrap.

gcc/
	* print-rtl.cc (print_exp): Avoid -Wempty-body warning.