.cargo: Define default linker for different targets

GCC and clang on Ubuntu tend to report different system library paths,
e.g.

```
$ gcc -print-search-dirs
[...]
libraries: =/usr/lib/gcc/x86_64-linux-gnu/11/:
/usr/lib/gcc/x86_64-linux-gnu/11/../../../../x86_64-linux-gnu/lib/x86_64-linux-gnu/11/:
/usr/lib/gcc/x86_64-linux-gnu/11/../../../../x86_64-linux-gnu/lib/x86_64-linux-gnu/:
/usr/lib/gcc/x86_64-linux-gnu/11/../../../../x86_64-linux-gnu/lib/../lib/:
/usr/lib/gcc/x86_64-linux-gnu/11/../../../x86_64-linux-gnu/11/:
/usr/lib/gcc/x86_64-linux-gnu/11/../../../x86_64-linux-gnu/:
/usr/lib/gcc/x86_64-linux-gnu/11/../../../../lib/:
/lib/x86_64-linux-gnu/11/:/lib/x86_64-linux-gnu/:
/lib/../lib/:/usr/lib/x86_64-linux-gnu/11/:/usr/lib/x86_64-linux-gnu/:
/usr/lib/../lib/:
/usr/lib/gcc/x86_64-linux-gnu/11/../../../../x86_64-linux-gnu/lib/:
/usr/lib/gcc/x86_64-linux-gnu/11/../../../:/lib/:/usr/lib/
```

```
$ clang -print-search-dirs
[...]
libraries: =/usr/lib/llvm-15/lib/clang/15.0.7:
/usr/lib/gcc/x86_64-linux-gnu/12:
/usr/lib/gcc/x86_64-linux-gnu/12/../../../../lib64:
/lib/x86_64-linux-gnu:/lib/../lib64:/usr/lib/x86_64-linux-gnu:
/usr/lib/../lib64:/lib:/usr/lib
```

In consequence, GCC and clang select different versions of libstdc++
when linking statically - GCC selects libstdc++ 11, clang selects
libstdc++ 12.

We build LLVM from source (`cargo xtask build-llvm`) using clang, which
causes our libLLVM artifacts to be linked to libstdc++ 12. Building
a static bpf-linker binary with GCC leads to linking errors, because
GCC attempts to statically link the older version of libstdc++.

To avoid such mismatches, and given that bpf-linker heavily relies on
LLVM ecosystem, use clang as a linker on all Linux targets.

Rust since 1.90.0 uses rust-lld as a default linker for
`x86_64-unknown-linux-gnu` target [0][1] and it does so by feeding
rust-lld into a C compiler, that is still used as a driver as a part of
`gnu-lld-cc` linker flavor (formerly implemented as `-Z gcc-ld=lld`
option)[2][3][4].

To benefit from faster linking times of lld (compared to bfd) on other
Linux targets, use `-fuse-ld` to force the usage of lld, that has to be
available in the system. Unfortunately, enabling rust-lld with
`-C linker-features=+lld`, `-C linker-flavor=gnu-lld-cc` or
`-C link-self-contained=+linker` is not stabilized. Using
`-C link-self-contained` (currently stabilized only with all options
enabled) does not work, as rustup toolchains do not provide all
necessary libraries for Linux targets. Neither does using rust-lld
directly due to lack of awarness about system search paths.

Use rust-lld directly on macOS without worrying about system search
paths.

[0] https://blog.rust-lang.org/2025/09/01/rust-lld-on-1.90.0-stable/
[1] rust-lang/rust#140525
[2] rust-lang/rust#71519
[3] rust-lang/rust#85961
[4] rust-lang/rust#112910

Author: vadorovsky

.cargo/config.toml

@@ -1,2 +1,34 @@
 [alias]
 xtask = "run --package xtask --"
+
+# On Linux we let clang drive the link so it contributes the right system
+# search paths.
+#
+# `x86_64-unknown-linux-gnu` target uses rust-lld as a default linker[0][1] and
+# it does so by feeding rust-lld into a C compiler, that is still used as a
+# driver as a part of `gnu-lld-cc` linker flavor (formerly implemented as
+# `-Z gcc-ld=lld` option)[2][3][4].
+#
+# [0] https://blog.rust-lang.org/2025/09/01/rust-lld-on-1.90.0-stable/
+# [1] https://github.com/rust-lang/rust/pull/140525
+# [2] https://github.com/rust-lang/rust/issues/71519
+# [3] https://github.com/rust-lang/rust/pull/85961
+# [4] https://github.com/rust-lang/rust/pull/112910
+[target.'cfg(target_os = "linux")']
+linker = "clang"
+
+# On other Linux targets we use `-fuse-ld` to force the usage of lld, that has
+# to be available in the system. Unfortunately, enabling rust-lld with
+# `-C linker-features=+lld`, `-C linker-flavor=gnu-lld-cc` or
+# `-C link-self-contained=+linker` is not stabilized. Using
+# `-C link-self-contained` (currently stabilized only with all options enabled)
+# does not work, as rustup toolchains do not provide all necessary libraries
+# for Linux targets. Neither does using rust-lld directly due to lack of
+# awarness about system search paths.
+[target.'cfg(all(target_os = "linux", any(not(target_arch = "x86_64"), not(target_env = "gnu"))))']
+rustflags = ["-C", "link-arg=-fuse-ld=lld"]
+
+# On macOS we can use rust-lld directly without worrying about system search
+# paths.
+[target.'cfg(target_os = "macos")']
+linker = "rust-lld"