implement dr_insert_get_seg_base() for AArch64 and add C analogue

From https://groups.google.com/g/dynamorio-users/c/piTkkQJAsHY

> > how can I get the tpidr_el0 system register value?
>
> I don't think there is a good solution today; the app's value is stolen and stored
> somewhere else and I don't think that's exposed to clients on AArch64. For x86,
> dr_insert_get_seg_base() is the way to get the base in generated code; I guess there
> isn't a way in C/C++ code (dr_get_dr_segment_base() is DR's). Probably
> dr_insert_get_seg_base() should be implemented for AArch64 to return the tpidr_el0
> app value, and perhaps an API routine should be added to to query the app base from C
> code (basically os_get_app_tls_base_offset(TLS_REG_LIB) internally). Could you file
> a feature request issue on this?
>