GCN: Fix 'real_from_integer' usage

GCN: Fix 'real_from_integer' usage

The recent commit b3f1b9e2aa079f8ec73e3cb48143a16645c49566
"build: Remove INCLUDE_MEMORY [PR117737]" exposed an issue in code added in
2020 GCN back end commit 95607c12363712c39345e1d97f2c1aee8025e188
"Zero-initialise masked load destinations"; compilation now fails:

    [...]
    In file included from ../../source-gcc/gcc/coretypes.h:507:0,
                     from ../../source-gcc/gcc/config/gcn/gcn.cc:24:
    ../../source-gcc/gcc/real.h: In instantiation of ‘format_helper::format_helper(const T&) [with T = std::nullptr_t]’:
    ../../source-gcc/gcc/config/gcn/gcn.cc:1178:46:   required from here
    ../../source-gcc/gcc/real.h:233:17: error: no match for ‘operator==’ (operand types are ‘std::nullptr_t’ and ‘machine_mode’)
       : m_format (m == VOIDmode ? 0 : REAL_MODE_FORMAT (m))
                     ^
    [...]

That's with 'g++ (GCC) 5.5.0', and seen similarly with
'g++ (Ubuntu 9.4.0-1ubuntu1~20.04.2) 9.4.0', for example.

	gcc/
	* config/gcn/gcn.cc (gcn_vec_constant): Fix 'real_from_integer'
	usage.