aarch64: Tabify sysdep-cancel.h
aarch64: Share code in syscall-cancel.h

Fold nocancel and error handling paths.
aarch64: Pass regno parameter to SINGLE_THREAD_P

Use cbz instead of cmp+b.eq.
aarch64: Improve syscall-cancel stack frame

One push instead of N; use stp/ldp to halve the insns.
aarch64: Use tpidr_el0 rather than __read_tp in librt
aarch64: Use tpidr_el0 rather than __errno_location in librt
aarch64: Rely on syscalls preserving registers

In several cases we've had asm routines rely on syscalls not clobbering
call-clobbered registers, and that's now deemed ABI.  So take advantage
of this in the INLINE_SYSCALL path as well.

Shrinks libc.so by about 1k.