stdbuf: fix cross-compilation

Summary:

Partial fix for https://github.com/uutils/coreutils/issues/6591

The current code declare libstdbuf as a build-dependency of stdbuf as a
workaround to enforce that libstdbuf is compiled before stdbuf. This breaks
cross-compilation, because build-dependencies were compiled for the host
architecture, and not for the target architecture.

The reason this workaround is necessary is that bindeps is available only in nightly at the moment:
https://rust-lang.github.io/rfcs/3028-cargo-binary-dependencies.html

This commit replaces the "build-dependency" workaround with another workaround:
calling cargo manually to build libstdbuf in the build.rs of stdbuf, in order to ensure that libstdbuf is built before stdbuf.

Changes:

- Removed cpp/cpp_build dependencies:

The cpp, cpp_build, and related dependencies were removed because they made cross-compilation in a build.rs file very complex, since you need
to pass proper CXX env variables for cross-compilation, whereas cross-compiling rust code using cargo is quite simple.
Provided Rust implementations for getting stdin, stdout, and stderr pointers.
Switched from C++/cpp macro-based initialization to using the Rust ctor crate for library initialization.

- Remove "feat_require_crate_cpp" which is not needed any more, since stdbuf was the only utility using the cpp crate.

Tests:

This commit fixes e.g. this test:
cross test --target aarch64-unknown-linux-gnu --features stdbuf test_stdbuf::test_libstdbuf_preload -- --nocapture

- The "i686" build of stdbuf was also broken (stdbuf 32 bits, but libstdbuf 64 bits) and test_stdbuf::test_libstdbuf_preload of the i686 builds in github CI serves as regression
test for this issue, no need to add a cross-rs test for aarch64.
- The x86_64 musl build of stdbuf was also broken and was passing tests in CI only because it was compiled with the wrong libc (glibc instead of musl)

Signed-off-by: Etienne Cordonnier <ecordonnier@snap.com>

.cargo/config.toml

@@ -3,3 +3,8 @@ linker = "x86_64-unknown-redox-gcc"
 
 [env]
 PROJECT_NAME_FOR_VERSION_STRING = "uutils coreutils"
+
+# libstdbuf must be a shared library, so musl libc can't be linked statically
+# https://github.com/rust-lang/rust/issues/82193
+[build]
+rustflags = [ "-C", "target-feature=-crt-static" ]