[AArch64][BZ 18648] change greg_t definition in ucontext.h

[AArch64][BZ 18648] change greg_t definition in ucontext.h

This is an ABI breaking change, but

	typedef int greg_t;

is not a useful definition on aarch64.

greg_t is usually used for defining gregset_t which is used
in mcontext_t.  The general registers in mcontext_t can only
be accessed by target specific code and on aarch64 greg_t
is not needed for that so this change is not supposed to break
existing code, just fix the definition.

	[BZ #18648]
	* sysdeps/unix/sysv/linux/aarch64/sys/ucontext.h (greg_t): Change the
	definition to elf_greg_t.

(Added another BZ entry that was missed in the previous commit).