Merge pull request #3885 from jjhursey/doc/v2.x/ppc-xl-pgi-note

README: Note about ld issue for XL and PGI on PPC

Author: jsquyres

README

@@ -193,6 +193,18 @@ Compiler Notes
   for more details:
   https://github.com/open-mpi/ompi/issues/3612
 
+- MPI applications that use the mpi_f08 module on PowerPC platforms
+  (tested ppc64le) will likely experience runtime failures if:
+   - they are using a GNU linker (ld) version after v2.25.1 and before v2.28,
+     -and-
+   - they compiled with PGI (tested 17.5) or XL (tested v15.1.5) compilers.
+  This was noticed on Ubuntu 16.04 which uses the 2.26.1 version of ld by
+  default. However, this issue impacts any OS using a version of ld noted
+  above. This GNU linker regression will be fixed in version 2.28.
+  Below is a link to the GNU bug on this issue:
+    https://sourceware.org/bugzilla/show_bug.cgi?id=21306
+  The XL compiler will include a fix for this issue in a future release.
+
 - On NetBSD-6 (at least AMD64 and i386), and possibly on OpenBSD,
   libtool misidentifies properties of f95/g95, leading to obscure
   compile-time failures if used to build Open MPI.  You can work