cmd/llvmplugin: describe pointer allocas as memory roots

[zhouguangyuan0718]

Summary

• replace pointer-alloca preload, gc-live, gc.relocate, and post-call write-back with a deopt pointer-map record
• enumerate supported fixed allocas into whole-object 64-bit slot bitmaps
• preserve existing deopt operands and ordinary SSA gc-live relocation
• keep writable callees authoritative over alloca memory, avoiding stale pointer restoration
• fail closed for dynamic, realigned, vector, non-entry, ambiguous select/PHI, volatile, atomic, and otherwise unstable allocas
• update the GoObj stack-map bridge, documentation, IR tests, malformed-contract tests, GoObj checks, and runtime GC/stack-growth fixture

Depends on goallc/llvm-project#18.

Validation

• GoALLC plugin CTest: 45/45 passed
• cmd/internal/testdir LLVM alloca-statepoint and runtime fixture passed
• ArgsPointerMaps, LocalsPointerMaps, PCDATA stack-map indexes, and frame offsets verified
• opt default O2 and llc verify-machineinstrs passed
• AArch64 wbZero/wbMove fixture produces no stale post-call store and no added root spill
• fresh-cache runtime GC and stack-growth executable passed

This is an independent replacement for the old fixed-home alloca model. It intentionally does not include the capture-analysis, readonly-special-case, or fake.use workaround from PR #32.