Fix matmul PR18857 and supply testcase

gcc/testsuite/ChangeLog

gcc/testsuite/gfortran.dg/matmul_1.f90

libcpp/aclocal.m4

libgfortran/ChangeLog

libgfortran/configure

libgfortran/generated/all_l4.c

libgfortran/generated/all_l8.c

libgfortran/generated/any_l4.c

libgfortran/generated/any_l8.c

libgfortran/generated/count_4_l4.c

libgfortran/generated/count_4_l8.c

libgfortran/generated/count_8_l4.c

libgfortran/generated/count_8_l8.c

libgfortran/generated/matmul_c4.c

libgfortran/generated/matmul_c8.c

libgfortran/generated/matmul_i4.c

libgfortran/generated/matmul_i8.c

libgfortran/generated/matmul_r4.c

libgfortran/generated/matmul_r8.c

libgfortran/m4/matmul.m4