cmd: qualify the LLVM backend on linux/amd64

[zhouguangyuan0718]

Summary

• qualify the LLVM backend tests on linux/amd64 for ABI pointer maps, fixed allocas, and write-barrier helpers
• mark AMD64 SSA-rewritten runtime.memmove and runtime.memequal calls with the same gc-leaf-function contract as dedicated LLVM lowering
• update X86 stack-map, PCSP, R_CALLIND, and memory-operation expectations
• allow and link zstd for Linux static LLVM builds
• update the vendored go-llvm dependency to v0.0.0-20260802061343-79fd6b1ee93f

Dependencies

• X86: complete GoObj statepoint and write barrier support llvm-project#27
• go-llvm: link zstd for static LLVM on Linux go-llvm#6

Validation

• make.bash -llvm-dir= -llvm-version=23 -llvm-link=dynamic
• make.bash -llvm-dir= -llvm-version=23 -llvm-link=static
• Test/LLVM codegen: 54/54 in both modes, including package init tasks
• ABI differential, alloca-statepoint, writebarrier-helpers, and writebarrier-ir
• dynamic runtime matrix: 10/10
• static runtime matrix: 10/10
• go test cmd/dist cmd/compile/internal/ssa cmd/objview cmd/llvmtoolexec
• go mod tidy -diff
• regenerated vendor tree matches the checked-in binding and modules.txt