You didn’t say exactly what the problem you’re seeing is, but I’m guessing that you’re crashing at the point of the call to printf. This is because OS X (both 32- and 64-bit) requires that the stack pointer have 16-byte alignment at the point of any external function call.
The stack pointer was 16-byte aligned when _main was called; that call pushed an eight-byte return address onto the stack, so the stack is not 16-byte aligned at the point of the call to _printf. Subtract eight from %rsp before making the call in order to properly align it.
So I went ahead and debugged this for you (no magic involved, just use gdb, break main, display/5i $pc, stepi, etc). The other problem you’re having is here:
movq _hello(%rip), %rdi
This loads the first eight bytes of your string into %rdi, which isn’t what you want at all (in particular, the first eight bytes of your string are exceedingly unlikely to constitute a valid pointer to a format string, which results in a crash in printf). Instead, you want to load the address of the string. A debugged version of your program is:
.cstring
_hello: .asciz "Hello, world\n"
.text
.globl _main
_main:
sub $8, %rsp // align rsp to 16B boundary
mov $0, %rax
lea _hello(%rip), %rdi // load address of format string
call _printf // call printf
add $8, %rsp // restore rsp
ret