[LLVM 11] Add SystemZ changes to release notes

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llvm/docs/ReleaseNotes.rst

 * A failure to insert indirect branches in position independent code
   was fixed.
 * The calculated expanded size of atomic pseudo operations was fixed, avoiding
   "fixup value out of range" errors during branch relaxation for some inputs.
 * The `mcountinhibit` CSR is now recognised.
 * The correct libcall is now emitted for converting a float/double to a 32-bit
   signed or unsigned integer on RV64 targets lacking the F or D extensions.
 
+Changes to the SystemZ Target
+-----------------------------
+
+* Added support for the MemorySanitizer and the LeakSanitizer.
+* Added support for the ``-fstack-clash-protection`` command line option.
+* Enhanced the assembler parser to allow using `%r0` even in an address
+  register context, and to allow specifying registers using plain integer
+  numbers instead of register names everywhere.
+* Fixed wrong code generation violating the platform ABI when passing
+  a C++ class (not struct) type having only a single member of
+  floating-point type.
+* Fixed wrong code generation when using the `vec_store_len_r` or
+  `vec_load_len_r` intrinsics with an immediate length argument of
+  16 or larger.
+* Miscellaneous codegen enhancements, in particular to improve vector code.
 
 Changes to the X86 Target
 -------------------------
 
 * Functions with the probe-stack attribute set to "inline-asm" are now protected
   against stack clash without the need of a third-party probing function and
   with limited impact on performance.
 * -x86-enable-old-knl-abi command line switch has been removed. v32i16/v64i8