This is the beginning of supporting minimum GLIBC version as part of the
target. See #2509 for the motivation.
The dummy libc zig files are removed. A future commit will build them
on-the-fly, using the generated text files generated by the new tool,
which are checked into source control and distributed along with zig.
These generated text files are, together, 142KB (20KB gzipped).
Compare that to a naive bundling of the .abilist files, which would be
2.2MiB (375KB gzipped).
This is based on glibc 2.29.
Unlike the other glibc source code checked into the repo, `csu/init.c`
did not have a license clause that allowed linking without restrictions.
`_IO_stdin_used` is the only symbol in the file and appears to be a 20
year old compatibility shim for the glibc 2.0 ABI. Obsolete in 2.1.
