[libunwind] [libunwind][AIX] Fix up TOC register if unw_getcontext is called from a different module (PR #66549)

[via cfe-commits]

================
@@ -305,9 +305,22 @@ DEFINE_LIBUNWIND_FUNCTION(__unw_getcontext)
   mflr  0
   std   0, PPC64_OFFS_SRR0(3) // store lr as ssr0
   PPC64_STR(1)
+  PPC64_STR(4)      // Save r4 first since it will be used for fixing r2.
+#if defined(_AIX)
+  // The TOC register (r2) was changed by the glue code if unw_getcontext
+  // is called from a different module. Save the original TOC register
+  // in the context if this is the case.
+  mflr  4
----------------
xingxue-ibm wrote:

Yes, `r0` has `LR` at this point and `mflr 4` has the same effect as `mr 4, 0` here. FWIW, I thought using `mflr 4` directly in the same new code block might make it a little easier for readers. If you think `mr 4, 0` is better, I can make the change.

https://github.com/llvm/llvm-project/pull/66549