SerenityOS: Add Platform module

Add an initial Platform module for SerenityOS [1]. This module is a mix
of the platform module currently used to build the Serenity Kernel and
Userspace applications and libraries, and the platform module included
in the CMake Port [2] which still has some work to do on the system
before its other patches could be considered for upstream.

As such, the platform module is currently only useful when used with a
suitably patched GCC or LLVM cross-compiler toolchain.

[1] https://github.com/SerenityOS/serenity
[2] https://github.com/SerenityOS/serenity/tree/master/Ports/cmake/patches

Issue: #23589
40 jobs for !6837 with add-serenityos-platform in 4 minutes and 43 seconds (queued for 3 seconds)
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