Found the source of non-reproducible builds from gfortran

Hi,

mrg mentioned that gfortran seems to be breaking reproducible builds, we
get references to the command line, and we use absolute paths(?), so a
failure may look as follows:

$ gfortran /tmp/hello.f90; strings --all a.out |grep tmp
/tmp/hello.f90

I came across the following unmerged patch submitted to GCC:
https://gcc.gnu.org/pipermail/gcc-patches/2020-July/549413.html

I can confirm that commenting out code in set_error_locus in
gcc/fortran/trans-io.cc seems to make this reference go away.