kbuild: rust: use -Zdebuginfo-compression

Rust 1.74.0 introduced (unstable) support for the
`-Zdebuginfo-compression` flag, thus use it.

Note that the releases built by the Rust project (i.e. the ones provided
by rustup) do not enable support for zstd in their bundled LLVM (yet,
at least), thus the Rust compiler will warn, but the build will proceed:

    warning: unknown debuginfo compression algorithm zstd - will fall
    back to uncompressed debuginfo

Link: rust-lang/rust#120953
Link: rust-lang/rust#115358
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240217002622.57322-1-ojeda@kernel.org
[ Added note about zstd support in Rust-provided binaries. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

scripts/Makefile.debug

@@ -35,10 +35,12 @@ endif
 
 ifdef CONFIG_DEBUG_INFO_COMPRESSED_ZLIB
 DEBUG_CFLAGS	+= -gz=zlib
+DEBUG_RUSTFLAGS	+= -Zdebuginfo-compression=zlib
 KBUILD_AFLAGS	+= -gz=zlib
 KBUILD_LDFLAGS	+= --compress-debug-sections=zlib
 else ifdef CONFIG_DEBUG_INFO_COMPRESSED_ZSTD
 DEBUG_CFLAGS	+= -gz=zstd
+DEBUG_RUSTFLAGS	+= -Zdebuginfo-compression=zstd
 KBUILD_AFLAGS	+= -gz=zstd
 KBUILD_LDFLAGS	+= --compress-debug-sections=zstd
 endif