chore: add mold linker config for aarch64-unknown-linux-gnu

Fixes ARM64 Docker build failure caused by R_AARCH64_CALL26 relocation
overflow when linking libv8. mold automatically generates range
extension thunks (veneers) to bridge calls exceeding the ±128MB limit.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/.cargo/config.toml

@@ -5,6 +5,10 @@ incremental = true
 linker = "clang"
 rustflags = ["-C", "link-arg=-fuse-ld=mold"]
 
+[target.aarch64-unknown-linux-gnu]
+linker = "clang"
+rustflags = ["-C", "link-arg=-fuse-ld=mold"]
+
 [target.x86_64-apple-darwin]
 rustflags = [
     "-C", "link-arg=-undefined",