Update release notes for 1.2.1

Author: msimberg

docs/sphinx/releases/whats_new_1_2_1.rst

@@ -16,17 +16,24 @@ General changes
 This is a bugfix release. It contains the following changes:
 
 * Fix compilation on ARM.
+* Fix compilation on s390x.
 * Fix compilation on 32-bit architectures.
 * Fix compilation with Boost 1.69.0.
 
 Closed issues
 =============
 
+* :hpx-issue:`3638` - Build HPX 1.2 with boost 1.69
 * :hpx-issue:`3550` - 1>e:\000work\hpx\src\throw_exception.cpp(54): error C2440: '<function-style-cast>': cannot convert from 'boost::system::error_code' to 'hpx::exception'
 * :hpx-issue:`3549` - HPX 1.2.0 does not build on i686, but release candidate did
+* :hpx-issue:`3511` - Build on s390x fails
 * :hpx-issue:`3509` - Build on armv7l fails
 
 Closed pull requests
 ====================
 
-* :hpx-pr:`3542` - Fix numa lookup from pu when using hwloc 2.x
+* :hpx-pr:`3648` - Adding timestamp option for S390x platform
+* :hpx-pr:`3647` - Blind attempt to fix warnings issued by gcc V9
+* :hpx-pr:`3591` - Fix compilation error on arm7 architecture. Compiles and runs on Fedora 29 on Pi 3.
+* :hpx-pr:`3558` - Adding constructor `exception(boost::system::error_code const&)`
+* :hpx-pr:`3551` - Fix uint64_t causing compilation fail on i686