Support for running qasan on aarch64 host

[cyanpencil]

Right now qasan does not support running aarch64 binaries on a aarch64 host. Having native support would be nice now that arm CPUs are gaining popularity.

Running ./qasan /bin/ls gives me the following crash:

qemu:handle_cpu_signal received signal outside vCPU context @ pc=0xaaaad6ce68c6

Tested on Ubuntu 18.04.1 LTS, aarch64, built with ./build.py --arch=aarch64 --cc gcc --cxx g++