runtime: support Go 1.26 stdlib calls from C libraries

[cpunion]

Problem

With Go 1.26, c-shared and c-archive exports that retain standard-library code need runtime hooks LLGo did not provide: Darwin CPU initialization needs the new sysctlbyname bridges, while Linux syscall needs gettimeofday, all-thread syscall, and cgo-caller symbols.

Linux c-shared also registered its constructor through llvm.global_ctors. The raw LLVM target machine lowers that to legacy .ctors, so LLD may not run the runtime and reflection-ABI initialization before C first calls an exported Go function.

Fix

• add the Darwin internal/cpu sysctl bridges through LLGo's dynamic-import trampoline;
• add the Linux/amd64 gettimeofday bridge and return ENOTSUP for thread-wide operations LLGo cannot safely implement;
• register only the Linux c-shared constructor in .init_array, retaining the existing platform lowering for c-archive and Darwin;
• call fmt.Sprintf, syscall.Gettimeofday, and syscall.AllThreadsSyscall from real static and dynamic C consumers.

The independent diff against current main is +151/-7 across 11 files.

Validation

• macOS arm64 / Go 1.26.5: c-shared and c-archive build, link, execute, and resolve Go DWARF source locations.
• Linux amd64 / Go 1.26.5 / LLVM 19.1.7: the same static and dynamic C consumers build, link, execute, and resolve DWARF; the Linux syscall checks run successfully.
• internal/build coverage is 73.6%; defineLibraryRuntimeInit is 100% covered on both platforms.
• No existing test is skipped or ignored.

Closes #2096.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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