aarch64: use <sys/user.h> defined register structures

glibc now supplies these (compatible) structs instead of including the
kernel's <asm/ptrace.h> header, so let's use them. Annoyingly this will
cause new elfutils to FTBFS on old glibc, and vice versa. So include a
new configure check for the new struct names and use the old ones if
they are not avilable.

Signed-off-by: Kyle McMartin <kyle@redhat.com>
Signed-off-by: Mark Wielaard <mjw@redhat.com>