README: update tested systems

These days Open MPI is being tested with PPC64 and ARMv8 reguarly. Move
these to the tested list. Also call out the version of GCC that is
reguarly tested.

Signed-off-by: Nathan Hjelm <hjelmn@google.com>

README

@@ -21,6 +21,7 @@ Copyright (c) 2017-2018 Los Alamos National Security, LLC.  All rights
                         reserved.
 Copyright (c) 2017      Research Organization for Information Science
                         and Technology (RIST). All rights reserved.
+Copyright (c) 2020      Google, LLC. All rights reserved.
 
 $COPYRIGHT$
 
@@ -174,8 +175,8 @@ General notes
   - Oracle Grid Engine (OGE) 6.1, 6.2 and open source Grid Engine
 
 - Systems that have been tested are:
-  - Linux (various flavors/distros), 64 bit (x86), with gcc, Absoft,
+  - Linux (various flavors/distros), 64 bit (x86, ppc, aarch64),
-    Intel, and Portland (*)
+    with gcc (4.8.x+), Absoft (fortran), Intel, and Portland (*)
   - macOS (10.12), 64 bit (x85_64) with XCode compilers
 
   (*) Be sure to read the Compiler Notes, below.
@@ -183,8 +184,8 @@ General notes
 - Other systems have been lightly (but not fully tested):
   - Linux (various flavors/distros), 32 bit, with gcc
   - Cygwin 32 & 64 bit with gcc
-  - ARMv6, ARMv7, ARMv8 (aarch64)
+  - ARMv6, ARMv7
-  - Other 64 bit platforms (e.g., Linux on PPC64)
+  - 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