how to compile glibc to pure arm instructions?

[Roland McGrath]

You'll have to ask the ARM maintainers and/or the people who wrote that
code.  My impression is that Thumb-2 has certain instructions not available
in ARM mode that can be used to produce better performance in some of these
hand-tuned assembly situations.  It's certainly the case that it is widely
assumed in userland that if the hardware supports Thumb-2 (e.g. any ARMv7
configuration), then the code is free to use it.