llvm-flang: Only build offload code if cuda enabled (#17466)

* llvm-flang Only build offload code if cuda enabled

The current version executes `cmake(*args)` always as part of the post install.  If device offload is not part of the build, this results in referencing `args` without it being set and the error:

```
==> Error: UnboundLocalError: local variable 'args' referenced before assignment

```

Looking at prevoous version of `llvm-package.py` this whole routine appears to be only required for offload, some indent `cmake/make/install` to be under the `if`.

* Update package.py

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1 files changed, 4 insertions, 3 deletions

diff --git a/var/spack/repos/builtin/packages/llvm-flang/package.py b/var/spack/repos/builtin/packages/llvm-flang/package.py
index 10001e2340..99948fd76f 100644
--- a/var/spack/repos/builtin/packages/llvm-flang/package.py
+++ b/var/spack/repos/builtin/packages/llvm-flang/package.py
@@ -238,6 +238,7 @@ class LlvmFlang(CMakePackage, CudaPackage):
                         spec['libelf'].prefix.include,
                         spec['hwloc'].prefix.include))
 
-        cmake(*args)
-        make()
-        make('install')
+            # Only build if offload target.
+            cmake(*args)
+            make()
+            make('install')