Cross-compile-o-naut wookey has recently posted a set of Debian & Ubuntu packages for an aarch64 (ie, 64-bit ARMv8) cross compiler:

The repositories here contain sources and binaries for the arm64 bootstrap in Debian (unstable) and Ubuntu (quantal). There are both toolchain and tools packages for amd64 build machines and arm64 binaries built with them. And corresponding sources.

For the lazy:

[jk@pecola ~]$ wget -O - |
    sudo apt-key add -
[jk@pecola ~]$ sudo apt-add-repository 'deb quantal-bootstrap main universe'
[jk@pecola ~]$ sudo apt-get update
[jk@pecola ~]$ sudo apt-get install --install-recommends gcc-4.7-aarch64-linux-gnu

... which gives you a cross compiler for aarch64:

[jk@pecola ~]$ cat helloworld.c 
#include <stdio.h>
int main(void) { printf("hello world\n"); return 0; }
[jk@pecola ~]$ aarch64-linux-gnu-gcc-4.7 -Wall -O2 -o helloworld helloworld.c
[jk@pecola ~]$ aarch64-linux-gnu-objdump -f helloworld 
helloworld:     file format elf64-littleaarch64
architecture: aarch64, flags 0x00000112:
start address 0x0000000000400450