These files are read by build.rs and turned into tests to
ensure that APIs aren't removed between libc releases.
File order
Files are including in the following order:
Family, e.g. unix.txt. NOTE: Windows is skipped here and includes as OS name
below.
Vendor, e.g. apple.txt. This allows us to have a single file with system
calls shared between multiple OSs, e.g. ios.txt, macos.txt share the same
kernel.
OS, e.g linux.txt, macos.txt, windows.txt.
Architecture specific system calls, e.g. linux-x86_64.txt or
linux-aarch64.txt.
Target environment, e.g. windows-mscv.txt or windows-gnu.txt.
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