[ARM] Small refactoring of arm gdbarch initialization

[ARM] Small refactoring of arm gdbarch initialization

This is in preparation to MVE support, where we will define another
pseudo register. We need to define the pseudo register numbers *after*
accounting for all the registers in the XML description, so move
the call to tdesc_use_registers up.

If we don't do it, GDB's register count won't consider registers contained
in the XML but ignored by GDB, throwing the register numbering off.