[ARM] CMSE code generation

This patch implements the final bits of CMSE code generation:

* emit special linker symbols

* restrict parameter passing to not use memory

* emit BXNS and BLXNS instructions for returns from non-secure entry
  functions, and non-secure function calls, respectively

* emit code to save/restore secure floating-point state around calls
  to non-secure functions

* emit code to save/restore non-secure floating-pointy state upon
  entry to non-secure entry function, and return to non-secure state

* emit code to clobber registers not used for arguments and returns
  when switching to no-secure state

Patch by Momchil Velikov, Bradley Smith, Javed Absar, David Green,
possibly others.

Differential Revision: https://reviews.llvm.org/D76518

Author: momchil-velikov

llvm/lib/Target/ARM/ARMAsmPrinter.cpp

@@ -73,6 +73,16 @@ void ARMAsmPrinter::emitFunctionEntryLabel() {
   } else {
     OutStreamer->emitAssemblerFlag(MCAF_Code32);
   }
+
+  // Emit symbol for CMSE non-secure entry point
+  if (AFI->isCmseNSEntryFunction()) {
+    MCSymbol *S =
+        OutContext.getOrCreateSymbol("__acle_se_" + CurrentFnSym->getName());
+    emitLinkage(&MF->getFunction(), S);
+    OutStreamer->emitSymbolAttribute(S, MCSA_ELF_TypeFunction);
+    OutStreamer->emitLabel(S);
+  }
+
   OutStreamer->emitLabel(CurrentFnSym);
 }