Optimize AArch64 handling of 32-bit personality

Optimize AArch64 handling of 32-bit personality

By putting aarch64_regs and arm_regs into a union,
register copying is eliminated.
No need to check and change personality on syscall exit.

* defs.h: Remove unused NUM_ARM_REGS define. Fix indentation.
* syscall.c: Put aarch64_regs and arm_regs into a union.
(update_personality): Shorten bitness message.
(printcall): Add commented-out PC printing.
(get_regs): Remove now-unnecessary 64-to-32 bits register copying.
(get_syscall_result): Drop personality changing code.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>