To get aarch64 rust-1.25.0 to build for my gentoo-on-rpi3-64bit project's binhost recently, I had to resort to emerging in a user-mode QEMU binfmt_misc chroot on my PC (it now isn't really feasible to build this natively on an RPi3, as it needs over 3GiB of swap (and isn't distcc-friendly), so the emerge time was literally stretching out to multiple days). Been a while since I tried this for aarch64 (gave up last time due to too many unimplemented syscalls), but I was pleasantly surprised to see that it now works pretty well.
As such, and since the technique (which lets you easily leverage the memory, CPU and peripherals bandwidth of your PC) may be of interest to others, I have written up a short wiki tutorial, here.
Here's a screenshot of the build in progress on my PC workstation (it hit 80 concurrent threads / 12 GiB memory at peak, incidentally ^-^):
This isn't a silver bullet unfortunately; user-mode QEMU has problems with certain programs, such as javac, atm (see e.g. this bug report), so no icedtea currently. But most things work.
Acknowledgement: I used the following articles as reference, thank you all: