[llvm-readtapi] Fix bad symlink with readtapi tool (llvm#68835)

The Clang-AIX-Wyvern-Dev buildbot is currently failing at the set
directory permission due to a bad symlink caused by this
[commit](https://github.ibm.com/compiler/llvm-project/commit/a54f31fabd55ab111484b1e094e3d2c6b29c4715).
This change is creating the additional symlink `readtapi ->
llvm-readtapi` which causes it to appear in
`/buildbot_worker/official-worker/clang-aix-dev/installdir`.

After some investigation, it looks like `readtapi` is the only tool that
is getting symlinked that is not guarded by the
`LLVM_INSTALL_BINUTILS_SYMLINKS` macro. For example:

1. `llvm/tools/llvm-objcopy/CMakeLists.txt`

```
if(LLVM_INSTALL_BINUTILS_SYMLINKS)
  add_llvm_tool_symlink(objcopy llvm-objcopy)
endif()
```

2. `llvm/tools/llvm-dwp/CMakeLists.txt`
```
if(LLVM_INSTALL_BINUTILS_SYMLINKS)
  add_llvm_tool_symlink(dwp llvm-dwp)
endif()
```

Author: nivetha-kuruparan

llvm/tools/llvm-readtapi/CMakeLists.txt

@@ -17,4 +17,6 @@ add_llvm_tool(llvm-readtapi
   ReadTAPIOptsTableGen
   )
 
-add_llvm_tool_symlink(readtapi llvm-readtapi)
+if(LLVM_INSTALL_BINUTILS_SYMLINKS)
+  add_llvm_tool_symlink(readtapi llvm-readtapi)
+endif()

llvm/utils/gn/secondary/llvm/tools/llvm-readtapi/BUILD.gn

@@ -0,0 +1,30 @@
+import("//llvm/tools/binutils_symlinks.gni")
+import("//llvm/utils/gn/build/symlink_or_copy.gni")
+
+if (llvm_install_binutils_symlinks) {
+  symlink_or_copy("readtapi") {
+    deps = [ ":llvm-readtapi" ]
+    source = "llvm-readtapi"
+    output = "$root_out_dir/bin/readtapi"
+  }
+}
+
+# //:llvm-readtapi depends on this symlink target, see comment in //BUILD.gn.
+group("symlinks") {
+  deps = [ ":llvm-readtapi" ]
+  if (llvm_install_binutils_symlinks) {
+    deps += [ ":readtapi" ]
+  }
+}
+
+executable("llvm-readtapi") {
+  deps = [
+    "//llvm/lib/Object",
+    "//llvm/lib/Support",
+    "//llvm/lib/TextAPI",
+  ]
+  sources = [
+    "DiffEngine.cpp",
+    "llvm-readtapi.cpp",
+  ]
+}