[AArch64] Fix pseudo-register numbering in the presence of unexpected additional...

[AArch64] Fix pseudo-register numbering in the presence of unexpected additional registers

When using AArch64 GDB with the QEMU debugging stub (in user mode), we get
additional system registers that GDB doesn't particularly care about, so
it doesn't number those explicitly.

But given the pseudo-register numbers are above the number of real registers,
we need to setup/account for the real registers first before going ahead and
numbering the pseudo-registers.  This has to happen at the end of
aarch64_gdbarch_init, after the call to tdesc_use_registers, as that
updates the total number of real registers.

This is in preparation to supporting pointer authentication for bare metal
aarch64 (QEMU).