[lldb][AArch64] Add release note for AArch64 Linux FPMR register

Author: DavidSpickett

llvm/docs/ReleaseNotes.md

@@ -268,6 +268,10 @@ Changes to the LLVM tools
 Changes to LLDB
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+* LLDB can now read the `fpmr` register from AArch64 Linux processes and core
+  files.
+
+
 Changes to BOLT
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