[patch, fortran] Reduce stack use in blocked matmul

[Thomas Koenig]

Am 10.05.2017 um 17:42 schrieb Thomas Koenig:
> 
> If you manage to come up with a legal Fortran testcas which
> sets b_dim1 to 0xdeadbeef, I owe you a beer :-)

... on a 32-bit system, of course.