Use x8 register for result struct address on aarch64

[tyfkda]

Unlike x64, aarch64 uses %x8 to pass the function result struct address:

typedef struct {
    long long x, y, z;
} FooStruct;

FooStruct return_struct(long long x, long long y, long long z) {
    FooStruct s = {.x = x, .y = y, .z = z};
    return s;
}

$ gcc -S -O2 -fno-asynchronous-unwind-tables ,foo/foo.c

	.section	__TEXT,__text,regular,pure_instructions
	.build_version macos, 13, 0	sdk_version 13, 3
	.globl	_return_struct                  ; -- Begin function return_struct
	.p2align	2
_return_struct:                         ; @return_struct
	.cfi_startproc
; %bb.0:
	stp	x0, x1, [x8]
	str	x2, [x8, #16]
	ret
	.cfi_endproc
                                        ; -- End function
.subsections_via_symbols