README: Remove Solaris 10/11 from tested platforms

Clarify exactly what compiler atomics support is needed to build Open
MPI.

For example, the following configuration now fails to configure on
Solaris:

- Studio 12.5 Sun C 5.14 SunOS_sparc 2016/05/31
- Also tried with GCC 4.0.2

with the following error:

configure: error: No atomic primitives available for sparc-sun-solaris2.11

Thanks to @jdelsign for the report.

Signed-off-by: Ralph Castain <rhc@pmix.org>
Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

README

@@ -15,7 +15,7 @@ Copyright (c) 2007      Myricom, Inc.  All rights reserved.
 Copyright (c) 2008-2018 IBM Corporation.  All rights reserved.
 Copyright (c) 2010      Oak Ridge National Labs.  All rights reserved.
 Copyright (c) 2011      University of Houston. All rights reserved.
-Copyright (c) 2013-2017 Intel, Inc. All rights reserved.
+Copyright (c) 2013-2020 Intel, Inc.  All rights reserved.
 Copyright (c) 2015      NVIDIA Corporation.  All rights reserved.
 Copyright (c) 2017-2018 Los Alamos National Security, LLC.  All rights
                         reserved.
@@ -186,8 +186,6 @@ General notes
   - Cygwin 32 & 64 bit with gcc
   - ARMv6, ARMv7
   - Other 64 bit platforms.
-  - Oracle Solaris 10 and 11, 32 and 64 bit (SPARC, i386, x86_64),
-    with Oracle Solaris Studio 12.5
   - OpenBSD.  Requires configure options --enable-mca-no-build=patcher
     and --disable-dlopen with this release.
   - Problems have been reported when building Open MPI on FreeBSD 11.1
@@ -213,6 +211,10 @@ Compiler Notes
 
 - Open MPI requires a C99-capable compiler to build.
 
+- On platforms other than x86-64, ARM, and PPC, Open MPI requires a
+  compiler that either supports C11 atomics or the GCC "__atomic"
+  atomics (i.e., GCC >= v4.7.2).
+
 - Mixing compilers from different vendors when building Open MPI
   (e.g., using the C/C++ compiler from one vendor and the Fortran
   compiler from a different vendor) has been successfully employed by