[libc++] Make `std::lock_guard` available with `_LIBCPP_HAS_NO_THREADS` #4831

Job	Run time
stage1 (generic-cxx03, clang-19, clang++-19)	7m 8s
stage1 (generic-cxx26, clang-19, clang++-19)	12m 27s
stage1 (generic-modules, clang-19, clang++-19)	10m 8s
stage1 (generic-gcc, gcc-14, g++-14)	16m 27s
stage2 (generic-cxx11, clang-19, clang++-19)	8m 37s
windows (clang-cl-dll, false)	16m 55s
stage2 (generic-cxx14, clang-19, clang++-19)	10m 36s
windows (clang-cl-static, false)	16m 57s
stage2 (generic-cxx17, clang-19, clang++-19)	9m 56s
windows (clang-cl-no-vcruntime, false)	16m 56s
stage2 (generic-cxx20, clang-19, clang++-19)	15m 25s
windows (clang-cl-debug, false)	16m 57s
stage2 (generic-cxx23, clang-19, clang++-19)	11m 34s
windows (clang-cl-static-crt, false)	16m 58s
stage2 (generic-gcc-cxx11, gcc-14, g++-14)	7m 36s
windows (mingw-dll, true)	16m 56s
stage2 (generic-cxx23, clang-17, clang++-17)	11m 3s
windows (mingw-static, true)	16m 58s
stage2 (generic-cxx26, clang-18, clang++-18)	12m 3s
windows (mingw-dll-i686, true)	16m 57s
stage3	1s
	4h 28m 35s

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