Drop external configuration for gnu_file_offset_bits64 / RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS #5199

Currently we have both RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS and RUST_LIBC_UNSTABLE_GNU_TIME_BITS (soon to be replaced with RUSTFLAGS cfg), with the intent that we support all combinations that glibc does on 32-bit platforms:

• 32-bit time_t, 32-bit off_t (default)
• 32-bit time_t, 64-bit off_t (#define _FILE_OFFSET_BITS 64)
• 64-bit time_t, 64-bit off_t ((#define _FILE_OFFSET_BITS 64, #define _TIME_BITS 64)

I don't expect the second option to be very useful to anyone and it's just one more thing to test/support, so I'm leaning toward dropping it before we start telling people they can use these config options. Of course, the internal cfg will stick around since it's helpful for mirroring the glibc source.

Cc @snogge for your thoughts since you authored all of this.