[patch, fortran] Reduce stack use in blocked matmul

[Thomas Koenig]

Hello world,

the attached patch reduces the stack usage by the blocked
version of matmul for cases where we don't need the full buffer.
This should improve stack usage.

Regression-tested.  I also added a stress test (around 3 secs of
CPU time on my system), it will only run once due to the "dg-do  run"
hack).

OK for trunk?

	Thomas

2017-05-05  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/80602
         * m4/matmul_internal.m4:  'matmul_name`:  Change
         t1 to a VLA of the required size.
         * generated/matmul_c10.c: Regenerated.
         * generated/matmul_c16.c: Regenerated.
         * generated/matmul_c4.c: Regenerated.
         * generated/matmul_c8.c: Regenerated.
         * generated/matmul_i1.c: Regenerated.
         * generated/matmul_i16.c: Regenerated.
         * generated/matmul_i2.c: Regenerated.
         * generated/matmul_i4.c: Regenerated.
         * generated/matmul_i8.c: Regenerated.
         * generated/matmul_r10.c: Regenerated.
         * generated/matmul_r16.c: Regenerated.
         * generated/matmul_r4.c: Regenerated.
         * generated/matmul_r8.c: Regenerated.

2017-05-05  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/80602
         * gfortran.dg/matmul_15.f90:  New test case.
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