GCN: Restore build with GCC 4.8

GCN: Restore build with GCC 4.8

For example, for "g++-4.8 (Ubuntu 4.8.4-2ubuntu1~14.04.4) 4.8.4", the recent
commit r13-3220-g45381d6f9f4e7b5c7b062f5ad8cc9788091c2d07
"amdgcn: add multiple vector sizes" broke the build:

    In file included from [...]/source-gcc/gcc/coretypes.h:458:0,
                     from [...]/source-gcc/gcc/config/gcn/gcn.cc:24:
    [...]/source-gcc/gcc/config/gcn/gcn.cc: In function ‘machine_mode VnMODE(int, machine_mode)’:
    ./insn-modes.h:42:71: error: temporary of non-literal type ‘scalar_int_mode’ in a constant expression
     #define QImode (scalar_int_mode ((scalar_int_mode::from_int) E_QImode))
                                                                           ^
    [...]/source-gcc/gcc/config/gcn/gcn.cc:405:10: note: in expansion of macro ‘QImode’
         case QImode:
              ^
    In file included from [...]/source-gcc/gcc/coretypes.h:478:0,
                     from [...]/source-gcc/gcc/config/gcn/gcn.cc:24:
    [...]/source-gcc/gcc/machmode.h:410:7: note: ‘scalar_int_mode’ is not literal because:
     class scalar_int_mode
           ^
    [...]/source-gcc/gcc/machmode.h:410:7: note:   ‘scalar_int_mode’ is not an aggregate, does not have a trivial default constructor, and has no constexpr constructor that is not a copy or move constructor
    [...]

Addressing this like simiar issues have been addressed in the past.

	gcc/
	* config/gcn/gcn.cc (VnMODE): Use 'case E_QImode:' instead of
	'case QImode:', etc.

(cherry picked from commit 612de72b0d2904b5a5a2b487ce4cb907c768a947)