Makefile: support universal macOS builds via RUST_TARGETS

On macOS, Universal Binaries contain native executable code for multiple architectures (such as Intel x86_64 and Apple Silicon arm64) bundled into a single file. This is standard practice for macOS distribution and CI packaging (such as internal distribution packages or tooling like Burrito/Homebrew), allowing a single build artifact to run natively across all Macs without Rosetta emulation or maintaining separate packages. When building Git C code for multiple architectures on macOS, the Apple toolchain (clang) natively supports universal builds via CFLAGS/LDFLAGS. When "-arch x86_64 -arch arm64" is passed, clang automatically compiles and links universal binaries for all C object files and executables out of the box. Cargo and rustc, however, do not support multiple "-arch" flags or emitting universal binaries in a single invocation. Instead, Cargo requires invoking each target triple independently (e.g., passing "--target x86_64-apple-darwin" and "--target aarch64-apple-darwin"). To bridge this gap when Rust is enabled: 1. Allow specifying space-separated target triples in RUST_TARGETS. 2. Introduce declarative pattern rules (target/%/...) to compile each target-specific library slice via Cargo. 3. On macOS, if multiple targets are specified, use "lipo" (part of the mandatory Xcode Command Line Tools) to combine the resulting static libraries into target/release/libgitcore.a. Once $(RUST_LIB) is compiled into a universal static archive, the standard C linker seamlessly links it with the C object files to produce universal Git executables. Signed-off-by: Shardul Natu <snatu@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>