Cherry picked from #4934 (cherry picked from commit 51ee690)

(cherry picked from commit bc0dd48)

[6.0] build: Repair the build on WASI platform

* Add stub for Testing module * Fix indentation

posix_spawn_file_actions_addchdir_np is the official way to set the working directory of a spawned process and is supported on both macOS and glibc (Linux, etc.). This makes Process.currentDirectoryURL thread-safe, as the current approach will result in the working directory being nondeterministically assigned when spawning processes across multiple threads, using different working directories.

Swift still needs to support Amazon Linux 2 until it EoLs in mid-2025. So restore the thread-unsafe fallback for systems with glibc older than version 2.29, which was removed in #4981.

The include path for libxml2 should be being set automatically by CMake because of the `target_link_libraries()` call; there is no need to add it explicitly. Additionally, it's a really bad idea if you specify an absolute path... rdar://137567628

…racterSet wrapper (#5107) (#5110) rdar://137806932 (cherry picked from commit cb6b2fa)

…#5115) (#5116) (cherry picked from commit bf7369a) Co-authored-by: noriaki watanabe <nabeyang@gmail.com>

… up to #5107) (#5120) rdar://138005684 (cherry picked from commit 592f015)

…ead-safe2 [6.0.x] Add a thread-safe implementation of Process.currentDirectoryURL

…der to avoid confusion over its linkage. Resolves#5125 (#5126) (#5127) (cherry picked from commit b1b4603)

…nsChdir. We need a handful more autolink arguments; these were in a previous PR, but seem to have gone missing at some point during the re-core. Additionally, Musl has the `posix_spawn_file_actions_addchdir_np()` function rather than the non-`_np()` version. Fixes#5089.

[Musl] Add back missing autolink arguments, fix CFPosixSpawnFileActionsChdir.