fix: make generated BPF toolchains analyzable on any CPU (#244)

## Problem

`declare_rustc_toolchains` gives the generated BPF toolchain a `linker`
select with no default branch, so the target cannot be analyzed for any
CPU other than `bpfeb`/`bpfel`. This reproduces against the stock
generated repo, no custom setup:

```
$ bazel cquery @default_rust_toolchains//rustc:all
ERROR: .../rustc/BUILD.bazel:5:25: configurable attribute "linker" in
@@rules_rs++toolchains+default_rust_toolchains//rustc:default_linux_x86_64_1_95_0_rust_toolchain_bpf
doesn't match this configuration. Would a default condition help?

Conditions checked:
  @@platforms//cpu:bpfeb
  @@platforms//cpu:bpfel
```

`bazel build` is unaffected because the toolchains are tagged `manual`,
but `manual` does not exempt a target from cquery. Anything that
analyzes a package rather than building it hits this — in our case
[target-determinator](https://github.com/bazel-contrib/target-determinator),
which made CI fail on a change that built fine locally.

## Fix

Both branches of the select held the same `bpf_linker_label`, so the
select can be dropped for the label itself. No behaviour change: BPF
targets are already gated by the toolchain's `target_settings`
(`@rules_rs//rs/toolchains:bpf_targets`), which is the right place for
that restriction.

`"//conditions:default": None` is not an option here — `rust_toolchain`
rejects it with *"When `rust_toolchain.linker_preference == \"rust\"`, a
`rust_toolchain.linker` must be provided"*.

## Verification

With this change, `bazel cquery @default_rust_toolchains//rustc:all`
completes successfully. Verified via `local_path_override` from a
workspace that registers the generated toolchains; its Rust targets
build and its tests pass unchanged.

I have no BPF targets to exercise, so the BPF path itself is only
covered by your CI.

## Related

Two other selects in the same macro have no default branch
(`target_triple` on the non-BPF toolchain, and the `{name}_rust_std`
alias). Those only bite when the macro is called from a workspace
package, and a naive default there would turn a loud analysis error into
a silently wrong target triple, so I have left them alone — happy to
file an issue with the details if that is useful.

Author: lsjostro

rs/toolchains/declare_rustc_toolchains.bzl

@@ -223,10 +223,12 @@ def declare_rustc_toolchains(
             process_wrapper = "@rules_rust//util/process_wrapper",
             rust_std = toolchain_rust_std,
             **(rust_toolchain_kwargs | {
-                "linker": select({
-                    "@platforms//cpu:bpfeb": bpf_linker_label,
-                    "@platforms//cpu:bpfel": bpf_linker_label,
-                }),
+                # Both branches of the former select held this same label, and
+                # a select with no default cannot be analyzed for any other
+                # CPU -- which breaks `bazel cquery` over the generated
+                # package.  BPF targets are already gated by the toolchain's
+                # target_settings.
+                "linker": bpf_linker_label,
             })
         )