[patch, libfortran] Adjust block size for libgfortran for unformatted reads

[Thomas Koenig]

Hello world,

the attached patch sets the I/O block size for unformatted files to
2**17 and makes this, and the block size for formatted files,
adjustable via environment variables.

The main reason is that -fconvert=big-endian was quite slow on
some HPC systems. A bigger buffer should eliminate that.  Also,
People who use unformatted files are likely to write large amounts
of data, so this seems like a good fit.  Finally, some benchmarking
showed that 131072 seemed like a good value to use. Thanks to Jerry
for support.

I didn't change the value for formatted files because, frankly, we are
using a lot of CPU for converting numbers there, so any gain
negligible (unless somebody comes up with a benchmark which says
otherwise).

As this is a change in behavior / new feature, I don't think that
backporting is indicated, but if somebody feels otherwise, please
speak up.

Regression-tested. OK for trunk?

Regards

	Thomas

2019-07-07  Thomas König  <tkoenig@gcc.gnu.org>

	PR libfortran/91030
	* gfortran.texi (GFORTRAN_BUFFER_SIZE_FORMATTED): Document
	(GFORTRAN_BUFFER_SIZE_FORMATTED): Likewise.

2019-07-07  Thomas König  <tkoenig@gcc.gnu.org>

	PR libfortran/91030
	* io/unix.c (BUFFER_SIZE): Delete.
	(BUFFER_SIZE_FORMATTED_DEFAULT): New variable.
	(BUFFER_SIZE_UNFORMATTED_DEFAULT): New variable.
	(unix_stream): Add buffer_size.
	(buf_read): Use s->buffer_size instead of BUFFER_SIZE.
	(buf_write): Likewise.
	(buf_init): Add argument unformatted.  Handle block sizes
	for unformatted vs. formatted, using defaults if provided.
	(fd_to_stream): Add argument unformatted in call to buf_init.
	* libgfortran.h (options_t): Add buffer_size_formatted and
	buffer_size_unformatted.
	* runtime/environ.c (variable_table): Add
	GFORTRAN_BUFFER_SIZE_UNFORMATTED and GFORTRAN_BUFFER_SIZE_FORMATTED.

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