build: fix C library DWARF defaults and runtime initialization

[cpunion]

Problem

c-shared and c-archive failed when LLGo applied its default DWARF omission. After restoring the builds, C calls into the libraries could still crash in runtime.Callers because runtime TLS had not been initialized. #2083 exposed both failures by making the C build and execution tests gating.

Fix

• support the same DWARF choices for c-shared and c-archive: LLGo default and explicit -w omit DWARF; explicit -w=false keeps it
• initialize the LLGo runtime from an llvm.global_ctors library constructor before exported Go functions run
• emit the constructor only when the library links the runtime

Coverage

• link-option tests cover default, -w, and -w=false for both C library modes
• macOS arm64 and Linux amd64 build, link, and run shared and static C consumers
• C calls verify runtime.Callers, CallersFrames, and FuncForPC return function, file, and line information
• explicit -w=false is checked with LLDB on macOS and llvm-dwarfdump on Linux, resolving captureCFuncInfo to export.go
• go test ./internal/build: 73.4% package coverage; changed link-option and constructor paths: 100%
• CI: 41 checks pass; the release publication job is skipped as expected

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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