Transmuting from an immutable to a mutable reference is undefined
behavior in Rust, with the exception of UnsafeCell which tells the
optimizer to not draw too many conclusions. Sadly UnsafeCell::get cannot
yet be used in statics (see https://github.com/rust-lang/rust/issues/51718),
so this works around that by making the statics into functions and
making C macros to call those.

Remove the broken Rust functions and instead provide C macros in a
`include/bits` header. The C macros were taken from musl.

The original implementation is modified from the `Cstr` method, but the
complexity there exists because it is trying to convert to the DST `Cstr`.

This is technically more correct, and matches the pointer methods in the
standard library.

Because we were previously converting the bytes in the format
string into Rust's char type and then printing that using the
format machinery, byte values that were not valid single-byte
UTF-8 characters failed to print correctly.  I found this while
trying to implement qsort() because the output of my test program
was mysteriously incorrect despite it working when I used glibc.