Add a disabled builder for aarch64 emulated tests

This commit adds a disabled builder which will run all tests for the standard
library for aarch64 in a QEMU instance. Once we get enough capacity to run this
on Travis this can be used to boost our platform coverage of AArch64

src/libstd/sys/unix/process/process_common.rs

@@ -469,6 +469,7 @@ mod tests {
     // although the reason isn't very clear as to why. For now this test is
     // ignored there.
     #[cfg_attr(target_arch = "arm", ignore)]
+    #[cfg_attr(target_arch = "aarch64", ignore)]
     fn test_process_mask() {
         unsafe {
             // Test to make sure that a signal mask does not get inherited.