[patch, fortran] Create temporary variables for matmul

[Dominique d'Humières]

I see a bunch of new failures with -m32

FAIL: gfortran.dg/allocatable_function_8.f90 -O0 execution test 
FAIL: gfortran.dg/allocatable_function_8.f90 -O1 execution test 
FAIL: gfortran.dg/allocatable_function_8.f90 -O2 execution test 
FAIL: gfortran.dg/allocatable_function_8.f90 -O3 -fomit-frame-pointer -funroll-loops -fpeel-loops -ftracer -finline-functions  execution test 
FAIL: gfortran.dg/allocatable_function_8.f90 -O3 -g execution test 
FAIL: gfortran.dg/allocatable_function_8.f90 -Os execution test 
FAIL: gfortran.dg/generic_20.f90 -O0 execution test 
FAIL: gfortran.dg/generic_20.f90 -O1 execution test 
FAIL: gfortran.dg/generic_20.f90 -O2 execution test 
FAIL: gfortran.dg/generic_20.f90 -O3 -fomit-frame-pointer -funroll-loops -fpeel-loops -ftracer -finline-functions execution test 
FAIL: gfortran.dg/generic_20.f90 -O3 -g execution test 
FAIL: gfortran.dg/generic_20.f90 -Os execution test 
FAIL: gfortran.dg/matmul_6.f90 -O0 execution test 
FAIL: gfortran.dg/matmul_bounds_5.f90 -O0 output pattern test 
FAIL: gfortran.dg/matmul_bounds_6.f90 -O0 execution test 
FAIL: gfortran.dg/matmul_bounds_6.f90 -O1 execution test 
FAIL: gfortran.dg/matmul_bounds_6.f90 -O2 execution test 
FAIL: gfortran.dg/matmul_bounds_6.f90 -O3 -fomit-frame-pointer -funroll-loops -fpeel-loops -ftracer -finline-functions execution test 
FAIL: gfortran.dg/matmul_bounds_6.f90 -O3 -g execution test 
FAIL: gfortran.dg/matmul_bounds_6.f90 -Os execution test 
FAIL: gfortran.dg/operator_1.f90 -O0 execution test 
FAIL: gfortran.fortran-torture/execute/intrinsic_matmul.f90 execution, -O0 

see, e.g., https://gcc.gnu.org/ml/gcc-regression/2017-05/msg00172.html. These failures result from the output of

Illegal instruction

at run time.

TIA

Dominique