This fixes an issue where Ghostty would not build against the macOS 15.5 SDK.
What was happening was that Zig was adding its embedded libc paths to
the clang command line, which included old headers that were
incompatible with the latest (macOS 15.5) SDK. Ghostty was adding the
newer paths but they were being overridden by the embedded libc paths.
The reason this was happening is because Zig was using its own logic to
find the libc paths and this was colliding with the paths we were
setting manually. To fix this, we now use a `libc.txt` file that
explicitly tells Zig where to find libc, and we base this on our own SDK
search logic.
This gets `zig build -Dtarget=aarch64-ios` working. By "working" I mean
it produces an object file without compiler errors. However, the object
file certainly isn't useful since it uses a number of features that will
not work in the iOS sandbox.
This is just an experiment more than anything to see how hard it would be to
get libghostty working within iOS to render a terminal. Note iOS doesn't
support ptys so this wouldn't be a true on-device terminal. The
challenge right now is to just get a terminal rendering (not usable).
