c: support Windows MSVC interop

[cpunion]

Summary

Add the Windows/MSVC host bindings needed by LLGo while preserving the existing POSIX C ABI on non-Windows hosts.

• add UCRT/MSVC implementations for standard streams, file/environment access, clocks, random, and setjmp
• add Win32 thread, FLS, synchronization, semaphore, and Winsock bindings
• select the Windows C++ runtime and cover the relevant LLVM 19 layouts
• reuse LLGo's native Windows synchronization backend instead of carrying a second lock implementation

The pthread_once boundary remains a direct C mapping:

int pthread_once(pthread_once_t *once_control, void (*init_routine)(void));

Accordingly, Once.Do accepts a C-ABI OnceFunc func() on every host. Capturing Go closures use the separate Once.DoFunc adapter: Go retains the closure object, while the native layer sees only a fixed callback and an opaque context pointer. The C layer never depends on LLGo's {function, environment} closure representation. The adapter covers concurrent callers and nested calls without changing the raw pthread API.

Validation

• go test ./... on macOS ARM64
• committed llgo test -v ./c/pthread/sync coverage for:
  • a raw non-capturing once callback
  • a capturing closure
  • concurrent callers with distinct closure contexts
  • nested DoFunc calls
• the LLGo tests pass through actual execution on macOS ARM64 and Windows 11 ARM64
• CI builds released LLGo v1.0.0 with LLVM 19 and runs the same tests on Linux and macOS
• Windows-target compile checks pass for the changed pthread/synchronization packages

The latest merged LLGo Windows foundation (R5) builds on Windows but cannot yet run llgo test: compiling the standard-library internal/fuzz package fails LLVM IR verification. Windows CI therefore waits for the remaining llgo test support to merge; the same tests already pass locally with the staged R8 integration.

The corresponding LLGo integration remains staged in xgo-dev/llgo#2425. This work is part of the Windows support proposal.

[cpunion]

Validation update:

• Added LLGo-native unit tests for the raw Once.Do callback, a captured DoFunc closure, concurrent callers with distinct contexts, and nested DoFunc calls.
• llgo test -v ./c/pthread/sync passes locally on macOS ARM64 and Windows 11 ARM64.
• The new Linux LLGo CI job passed all four tests using released LLGo v1.0.0 and LLVM 19: https://github.com/goplus/lib/actions/runs/33037946987/job/98404795022
• The ordinary Go build job remains green.

[cpunion]

LLGo execution coverage is now automated on both host platforms:

• Ubuntu: passed in 1m29s
• macOS: passed in 2m46s

Both jobs build LLGo v1.0.0 with LLVM 19 and run all four c/pthread/sync tests.

Windows 11 ARM64 has also passed the same tests locally with the staged R8 integration. I additionally built and tested the latest merged LLGo Windows commit (71754b459, R5): LLGo itself builds, but llgo test still fails while compiling standard-library internal/fuzz due invalid LLVM IR. Windows CI should therefore be added after the remaining R6 testing support is merged, rather than pinning this upstream PR to an unmerged fork commit.