Add configure check for arm64 user_regs_struct.

old glibc defined kernel user_pt_regs, but newer glibc instead
define user_regs_struct. Add a configure test to see what we need.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14176 a5019735-40e9-0310-863c-91ae7b9d1cf9