aarch64: fix errno address calculation in SYSCALL_ERROR_HANDLER

This patch fixes the last regression in LTP lite scenario (mmap16) comparing
to lp64 in my source trees [1, 2]. The fix has been suggested back in 2015 [3]
but was never applied.

Checked on aarch64-linux-gnu.

	* sysdeps/unix/sysv/linux/aarch64/sysdep.h: use PTR_REG() for offset
	calculation in SYSCALL_ERROR_HANDLER().

[1] https://github.com/norov/glibc/tree/dev9
[2] https://github.com/norov/linux/tree/ilp32-20170203
[3] https://sourceware.org/ml/libc-alpha/2015-03/msg00587.html