[AArch64] Stop reserved registers from being saved in prolog/epilog

GCC's man page is clear on how -ffixed-reg must behave:
```
  Treat the register named reg as a fixed register; generated
  code should never refer to it (except perhaps as a stack pointer,
  frame pointer or in some other fixed role).
```

This implies prolog/epilog code also must not save/restore explicitly
fixed registers, even when it is callee-saved. Some projects rely on
this (GCC's) behavior.

For example,
```
void f() {
  register uint64_t x28 asm("x28") = 0xee;
  asm volatile("" : "+r"(x28)); // avoid mov being eliminated
}
```
should not touch x28 outside of `mov w28,#0xee`.

For riscv64 clang behaves the same as GCC.

Fixes #111379.

Author: yasuna-oribe

llvm/lib/Target/AArch64/AArch64FrameLowering.cpp

@@ -3619,6 +3619,14 @@ void AArch64FrameLowering::determineCalleeSaves(MachineFunction &MF,
     if (Reg == BasePointerReg)
       SavedRegs.set(Reg);
 
+    // Don't save fixed registers specified with -ffixed-reg.
+    if (!produceCompactUnwindFrame(MF) &&
+        AArch64::GPR64RegClass.contains(Reg) &&
+        RegInfo->isReservedReg(MF, Reg)) {
+      SavedRegs.reset(Reg);
+      continue;
+    }
+
     bool RegUsed = SavedRegs.test(Reg);
     unsigned PairedReg = AArch64::NoRegister;
     const bool RegIsGPR64 = AArch64::GPR64RegClass.contains(Reg);