aarch64: Revert the change of the __reserved member of mcontext_t

aarch64: Revert the change of the __reserved member of mcontext_t

The uc_mcontext.__reserved member of ucontext_t is a user visible API,
that should not be changed, because this is the only way to access cpu
states of various extensions of linux asm/sigcontext.h, it does not
violate namespace rules either, so revert this part of the commit

commit 4fa9b3bfe6759c82beb4b043a54a3598ca467289
Commit:     Joseph Myers <joseph@codesourcery.com>

    Fix mcontext_t sigcontext namespace (bug 21457).

(In principle the user can type cast &uc_mcontext to struct sigcontext*
to use the linux sigcontext fields, but that's not the existing practice
since mcontext_t used to be a typedef of struct sigcontext.)

	[BZ #22742]
	* sysdeps/unix/sysv/linux/aarch64/sys/ucontext.h (__glibc_reserved1):
	Rename to __reserved and add comment.
	* sysdeps/unix/sysv/linux/aarch64/ucontext_i.sym (__glibc_reserved1):
	Rename to __reserved.