rust: jump_label: skip arch-specific asm in testlib builds

Running `make rusttest` with `ARCH=` set to an architecture other than
the host's may fail in the future, e.g. `ARCH=arm64` on an x86_64 host:

error: alignment must be a power of 2
  --> rust/kernel/jump_label.rs:51:13
   |
51 |     include!(concat!(env!("OBJTREE"), "/rust/kernel/generated_arch_static_branch_asm.rs"));
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: instantiated into assembly here
  --> <inline asm>:3:10
   |
 3 |     .align        3
   |                   ^

The reason is that `rusttest` builds the kernel crate as a host
library: it passes the `CONFIG_*` cfgs of the configured architecture,
but not `--target`, so code generation happens for the
host. `arch_static_branch!` then selects the arch-specific inline asm
arm based on CONFIG_*, and the host assembler rejects it.

This does not happen with the current master because
`arch_static_branch!` has no user inside the kernel crate itself yet,
but fix it now to avoid surprises later.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Link: https://patch.msgid.link/20260809134858.1219036-1-tomo@flapping.org
[ Reworded slightly to clarify it "may fail in the future". - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/jump_label.rs

@@ -44,6 +44,7 @@ macro_rules! static_branch_unlikely {
 
 #[macro_export]
 #[doc(hidden)]
+#[cfg(not(testlib))]
 #[cfg(CONFIG_JUMP_LABEL)]
 macro_rules! arch_static_branch {
     ($key:path, $keytyp:ty, $field:ident, $branch:expr) => {'my_label: {
@@ -61,6 +62,17 @@ macro_rules! arch_static_branch {
     }};
 }
 
+#[macro_export]
+#[doc(hidden)]
+#[cfg(testlib)]
+#[cfg(CONFIG_JUMP_LABEL)]
+macro_rules! arch_static_branch {
+    ($key:path, $keytyp:ty, $field:ident, $branch:expr) => {
+        // The asm falls through until patched, which never happens on the host.
+        false
+    };
+}
+
 #[cfg(CONFIG_JUMP_LABEL)]
 pub use arch_static_branch;