aarch64: fix compilation warnings

aarch64: fix compilation warnings

Fix compilation warnings introduced by commit
5b9b7e1d347eb4556084cdccad75e8247c535ed5:

signal.c:702:45: warning: signed and unsigned type in conditional expression [-Wsign-compare]
syscall.c:746:34: warning: initialization from incompatible pointer type [enabled by default]

* syscall.c [AARCH64] (aarch64_sp_ptr): Add explicit cast.
[AARCH64] (arm_sp_ptr): Change pointer type to unsigned.
* linux/aarch64/arch_regs.h (arm_sp_ptr): Update.

Reported-by: Mike Frysinger <vapier@gentoo.org>