libcxx presubmit should have bootstrapping-build variant for mac and windows, not just linux

[zeroomega]

Most of current libcxx presubmit builders uses the host clang, which is the current release version. There is a special bootstraping-build presubmit builder that builds the ToT Clang and used it to build and test libcxx (e.g.). However, this builder only covers the Linux platform and it missed following breaking changes in the past:

• 10c6d63 and 987087d (which broke llvm-libc++-static-clangcl.cfg.in :: libcxx/fuzzing/random.pass.cpp on Windows)
• PR [libc++][TZDB] Implements time_zone::to_sys. #90394 which broke chrono.compile.pass.cpp on Windows
• PR [libc++][modules] Adds module testing.  #76246 which broke libcxx/selftest/modules/std-and-std.compat-module.sh.cpp on Windows

By supporting the Mac and Windows in the bootstraping-build builder, these failures could be caught, reducing downstream disturbances.