[X86] Update release notes for -mtune support.

Author: topperc

clang/docs/ReleaseNotes.rst

@@ -190,6 +190,11 @@ X86 Support in Clang
 
 - Support for -march=sapphirerapids was added.
 
+- The -mtune command line option is no longer ignored for X86. This can be used
+  to request microarchitectural optimizations independent on -march. -march=<cpu>
+  implies -mtune=<cpu>. -mtune=generic is the default with no -march or -mtune
+  specified.
+
 Internal API Changes
 --------------------
 

llvm/docs/ReleaseNotes.rst

@@ -101,6 +101,11 @@ During this release ...
 * The assembler now has support for {disp32} and {disp8} pseudo prefixes for
   controlling displacement size for memory operands and jump displacements. The
   assembler also supports the .d32 and .d8 mnemonic suffixes to do the same.
+* A new function attribute "tune-cpu" has been added to support -mtune like gcc.
+  This allows microarchitectural optimizations to be applied independent from
+  the "target-cpu" attribute or TargetMachine CPU which will be used to select
+  Instruction Set. If the attribute is not present, the tune CPU will follow
+  the target CPU.
 
 Changes to the AMDGPU Target
 -----------------------------