cmd/compile: keep defer results live at safepoints

[zhouguangyuan0718]

Summary

• mark pointer-containing named result homes in functions with defer and keep them in every statepoint gc-live set
• emit volatile result reloads in defer recovery successors so O2 retains the alloca home
• use the existing alloca deopt pointer-map protocol and verify that no StackObject is emitted when the result is live at every call
• qualify fixedbugs/issue27518b on Darwin/arm64

This depends on goallc/llvm-project#43 because O2 may merge the normal and recovery callbr destinations.

Validation

• rebuilt the GoALLC toolchain with the LLVM cmd: qualify the LLVM backend on linux/amd64 #43 build
• all 58 GoALLC statepoint plugin tests pass
• targeted LLVM codegen and runtime tests pass under default
• fixedbugs/issue27518b passes through the whitelist harness and ran successfully 10 consecutive times
• GoObj inspection confirms the result slot bit is present at runtime.panicmem and no StackObjects record is emitted
• cmd/compile/internal/ssa, cmd/compile/internal/gc, and cmd/llvmtoolexec tests pass