mlibc is a C standard library

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Design of the library

Directory	Purpose
`options/`	(More or less) OS-independent headers and code. `options/` is divided into subdirectories that can be enabled or disabled by ports.
`sysdeps/`	OS-specific headers and code. `sysdeps/` is divded into per-port subdirectories. Exactly one of those subdirectories is enabled in each build.
`abis/`	OS-specific interface headers ("ABI headers"). Those contain the constants and structs of the OS interface. For example, the numerical values of `SEEK_SET` or `O_CREAT` live here, as well as structs like `struct stat`. ABI headers are only allowed to contain constants, structs and unions but no function declarations or logic. `abis/` is divided into per-OS subdirectories but this division is for organizational purposes only. Ports can still mix headers from different `abis/` subdirectories.

Porting mlibc to a new OS: Ports to new OSes are welcome. To port mlibc to another OS, the following changes need to be made:

Add new sysdeps/ subdirectory sysdeps/some-new-os/ and a meson.build to compile it. Integreate sysdeps/some-new-os/meson.build into the toplevel meson.build.
Create ABI headers in abis/some-new-os/. Add symlinks in sysdeps/some-new-os/include/abi-bits to your ABI headers. Look at existing ports to figure out the ABI headers required for the options enabled by sysdeps/some-new-os/meson.build.
In sysdeps/some-new-os/, add code to implement (a subset of) the functions from options/internal/include/mlibc/sysdeps.hpp. Which subset you need depends on the options that sysdeps/some-new-os/meson.build enables.