Trac #23272: OpenBLAS - alterations for TARGET

On purchasing a new laptop, I attempted to MAKE sage as I normally have
and ran into the following error:

{{{

Error building Sage.

The following package(s) may have failed to build (not necessarily
during this run of 'make all'):

* package: openblas-0.2.19.p0
  log file: /sage/sage/logs/pkgs/openblas-0.2.19.p0.log
  build directory:
/sage/sage/local/var/tmp/sage/build/openblas-0.2.19.p0
}}}

The error seemed to be really coming from:

{{{                                               ^
make[3]: *** [getarch_2nd] Error 1
Makefile:123: *** OpenBLAS: Detecting CPU failed. Please set TARGET
explicitly, e.g. make TARGET=your_cpu_target. Please read README for the
detail..  Stop.
make[3]: Leaving directory
'/sage/sage/local/var/tmp/sage/build/openblas-0.2.19.p0/src'
Error building OpenBLAS
}}}

It turned out that this error has been seen multiple times in Sage:
* https://groups.google.com/forum/#!topic/sage-devel/zQsZsivts0I
* https://groups.google.com/forum/#!topic/sage-release/3QJoAgg9bgo
* https://groups.google.com/d/msg/sage-devel/FhPQS_R_CTA/g-yQFXXzAgAJ
* https://groups.google.com/d/msg/sage-devel/-Exvx0bane8/9kC8hBEFBwAJ
* https://groups.google.com/d/msg/sage-devel/JtDS2kHafAA/DWdHpWPxAgAJ

etc.

I noticed that updating the config (as mentioned in the threads above)
accurately solved the problem, but I wanted to try and fix the problem
head on rather than just temporarily solve it. With that in mind, I
conversed with the OpenBLAS people:
OpenMathLib/OpenBLAS#1204
and they suggested I update the cpu file for the build, which I have
included.

Additionally, I wanted to fix this problem for future users and thus I
wanted to add a fail-safe for the code so that if this error happens
again, then we automatically try the TARGET=ATOM method. But, I want to
create this with the caveat that a user can change the method.

Before finalizing these changes, we should note a few things, but as
I've not worked that long with sage I don't know how to address them. In
essence:

 - Are the changes I made appropriate for the make?
 - I'm adding documentation on this as there is no documentation.
 - There was questions on why we have "p0" at the end of our version? I
assume this is from the patch, but I don't know.
 - The programmer suggested we change the " top level option to use
-lblas (debian/ubuntu) -lopenblas (suse/fedora/epel)". I don't know what
that means in regards to us, but figured someone might be able to answer
this question and maybe we can ensure this is happening.

If there are any questions feel free to ask, and if you believe this is
not necessarily or if something else is better practice, let me know.
I've never done a bash alteration so not sure what the details for that.

The tarball is here
https://github.com/xianyi/OpenBLAS/archive/v0.2.20.tar.gz
(using the browser to download it will give it correct name)

URL: https://trac.sagemath.org/23272
Reported by: aram.dermenjian
Ticket author(s): Aram Dermenjian
Reviewer(s): Volker Braun, Dima Pasechnik

Author: Release Manager

build/pkgs/openblas/checksums.ini

@@ -1,4 +1,4 @@
 tarball=OpenBLAS-VERSION.tar.gz
-sha1=df4ceb25d2529748ad0f2715e62fefb348477edd
-md5=28c998054fd377279741c6f0b9ea7941
-cksum=3048490140
+sha1=a186074145a24823e82c65672dad1cd1ca6fe89c
+md5=48637eb29f5b492b91459175dcc574b1
+cksum=607023019

build/pkgs/openblas/package-version.txt

@@ -1 +1 @@
-0.2.19.p0
+0.2.20.p1