Depends on #2090

Problem

Go 1.26 standard-library code retained by c-shared and c-archive needs runtime hooks LLGo did not provide. This caused unresolved symbols on Darwin. On Linux, the generated shared-library constructor could fail to initialize the runtime and reflection ABI tables before C first called an exported Go function.

Fix

• add the missing Go 1.26 synctest, Darwin sysctl, and Linux syscall/time runtime hooks;
• initialize the runtime and reflection ABI tables before C calls Go;
• emit the Linux c-shared constructor in .init_array, while retaining platform constructor lowering for c-archive and Darwin;
• exercise fmt.Sprintf, goroutines, funcinfo, platform syscall paths, and DWARF lookup from real C consumers.

After rebasing onto current main, the Darwin sysctl additions extend os_darwin.go instead of duplicating the bridge already merged by #2150.

This PR has an independent diff of +187/-25 across 11 files. The broader standard-library package matrix remains in #2134.

Validation

• macOS arm64 / Go 1.26.5: focused build tests and real LLGo runtime/debug execution pass.
• Ubuntu arm64 / Go 1.26.5 / LLVM 19.1.7: executable, c-shared, and c-archive C runners pass in an 8 GiB, 2 CPU, no-swap container.
• TestDefineLibraryRuntimeInitInitializesAbiTypes covers constructor call order.
• No existing test is skipped or ignored.

Closes #2096.